diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 323f63544..a709ff3ea 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -23467,9 +23467,11 @@ paths:
         - in: query
           name: values
           schema:
-            description: Comma-separated value series to include, as `MeasurementType` names or `MOOD`.
+            description: Comma-separated value series to include, as `MeasurementType` names or `MOOD`, at most 6. More, or an
+              unknown name, answers 422.
             type: string
-          description: Comma-separated value series to include, as `MeasurementType` names or `MOOD`.
+          description: Comma-separated value series to include, as `MeasurementType` names or `MOOD`, at most 6. More, or an
+            unknown name, answers 422.
       responses:
         "200":
           description: The timeline for the requested zoom and window.
@@ -81541,6 +81543,14 @@ components:
           items:
             $ref: "#/components/schemas/TimelineStandingItem"
           description: "Things without a start: shown once as standing."
+        bucket:
+          type: string
+          enum:
+            - quarter
+            - month
+            - week
+          description: "The span every series point averages: `quarter` for a multi-year `all`, `month` for `year` and an `all`
+            under two years, `week` for `quarter`."
         series:
           type: array
           items:
@@ -81569,6 +81579,7 @@ components:
         - range
         - lanes
         - standing
+        - bucket
         - series
         - notable
       additionalProperties: false
@@ -81683,12 +81694,6 @@ components:
           anyOf:
             - type: string
             - type: "null"
-        granularity:
-          type: string
-          enum:
-            - month
-            - week
-            - day
         points:
           type: array
           items:
@@ -81696,17 +81701,25 @@ components:
             properties:
               t:
                 type: string
-                description: A local calendar date, `YYYY-MM-DD`.
+                description: The bucket's first day.
               mean:
                 type: number
+                description: The mean of the bucket's day values, each day weighing one (a cumulative type's days are its day totals, a
+                  pulse-like type's its hourly-mean day value).
+              count:
+                type: integer
+                minimum: 1
+                maximum: 9007199254740991
+                description: Readings behind the mean. Clients draw a point from fewer than three as thin.
             required:
               - t
               - mean
+              - count
             additionalProperties: false
+          description: Buckets with at least one reading, oldest first. A bucket without one is absent, never interpolated.
       required:
         - key
         - unit
-        - granularity
         - points
       additionalProperties: false
     TimelineReadinessEnvelope:
diff --git a/src/app/api/timeline/route.ts b/src/app/api/timeline/route.ts
index 6180427b2..a14be92d3 100644
--- a/src/app/api/timeline/route.ts
+++ b/src/app/api/timeline/route.ts
@@ -45,7 +45,7 @@ export const GET = apiHandler(async (request: Request) => {
   }
   const seriesKeys = parseSeriesKeys(parsed.data.values);
   if (seriesKeys === null) {
-    return apiError("Unknown value series", 422, {
+    return apiError("Unknown value series, or more than six", 422, {
       errorCode: "timeline.invalid",
     });
   }
diff --git a/src/lib/day/contract.ts b/src/lib/day/contract.ts
index 500d8d2f2..7b3c419fd 100644
--- a/src/lib/day/contract.ts
+++ b/src/lib/day/contract.ts
@@ -212,6 +212,22 @@ export const TIMELINE_ZOOMS = ["all", "year", "quarter"] as const;
 
 export type TimelineZoom = (typeof TIMELINE_ZOOMS)[number];
 
+/**
+ * The span one value-series point averages. The server picks it per zoom
+ * (`timelineBucket` in `src/lib/timeline/load-timeline.ts`): quarters for a
+ * multi-year `all`, months for `year` and a short `all`, weeks for `quarter`.
+ */
+export const TIMELINE_BUCKETS = ["quarter", "month", "week"] as const;
+
+export type TimelineBucket = (typeof TIMELINE_BUCKETS)[number];
+
+/**
+ * The most value series one request may name. Each series is its own row
+ * under the lanes, drawn in neutral ink and labelled at the left, so the cap
+ * is the chart's height, not a palette: six rows add 360 px under the lanes.
+ */
+export const TIMELINE_MAX_SERIES = 6;
+
 /**
  * The lanes, top to bottom. An empty lane is not sent. Each lane a module
  * owns is a surface (`timeline-lane:<key>`) and disappears with it.
diff --git a/src/lib/day/daily-stats.ts b/src/lib/day/daily-stats.ts
index 0645b57dc..a9bcba033 100644
--- a/src/lib/day/daily-stats.ts
+++ b/src/lib/day/daily-stats.ts
@@ -30,6 +30,15 @@ import { collapseRollupRowsBySource } from "@/lib/rollups/measurement-read";
 /** Day key → the day's value, per type. */
 export type DailyStats = Map<MeasurementType, Map<string, number>>;
 
+/** A day's value with the readings behind it (the picked source's). */
+export interface DayCell {
+  value: number;
+  count: number;
+}
+
+/** Day key → the day's value and reading count, per type. */
+export type DailyCells = Map<MeasurementType, Map<string, DayCell>>;
+
 interface StatRow {
   type: MeasurementType;
   source: MeasurementSource;
@@ -60,7 +69,22 @@ export async function readLocalDailyStats(args: {
   tz: string;
   priorityJson: unknown;
 }): Promise<DailyStats> {
-  const out: DailyStats = new Map();
+  return valuesOnly(await readLocalDailyCells(args));
+}
+
+/**
+ * {@link readLocalDailyStats} with each day's reading count beside its
+ * value: the timeline marks a mean that rests on few readings.
+ */
+export async function readLocalDailyCells(args: {
+  userId: string;
+  types: readonly MeasurementType[];
+  from: Date;
+  to: Date;
+  tz: string;
+  priorityJson: unknown;
+}): Promise<DailyCells> {
+  const out: DailyCells = new Map();
   if (args.types.length === 0 || args.from >= args.to) return out;
   const hourlyTypes = args.types.filter((t) => HOURLY_MEAN_DAY_TYPES.has(t));
   // Every value is parameter-bound; the enum casts are literal SQL. The hour
@@ -87,7 +111,17 @@ export async function readLocalDailyStats(args: {
       AND "measured_at" < ${args.to}
     GROUP BY 1, 2, 3, 4
   `;
-  return foldDailyStats(rows, args.priorityJson);
+  return foldDailyCells(rows, args.priorityJson);
+}
+
+function valuesOnly(cells: DailyCells): DailyStats {
+  const out: DailyStats = new Map();
+  for (const [type, days] of cells) {
+    const perDay = new Map<string, number>();
+    for (const [day, cell] of days) perDay.set(day, cell.value);
+    out.set(type, perDay);
+  }
+  return out;
 }
 
 /**
@@ -99,6 +133,14 @@ export function foldDailyStats(
   rows: readonly StatRow[],
   priorityJson: unknown,
 ): DailyStats {
+  return valuesOnly(foldDailyCells(rows, priorityJson));
+}
+
+/** {@link foldDailyStats} with the picked source's reading count per day. */
+export function foldDailyCells(
+  rows: readonly StatRow[],
+  priorityJson: unknown,
+): DailyCells {
   // type → day → source → hour cells
   const nested = new Map<
     MeasurementType,
@@ -114,7 +156,7 @@ export function foldDailyStats(
     cells.push({ total: Number(row.total), count: Number(row.count) });
   }
 
-  const out: DailyStats = new Map();
+  const out: DailyCells = new Map();
   for (const [type, days] of nested) {
     const cumulative = isCumulativeDaySumType(type);
     const hourly = HOURLY_MEAN_DAY_TYPES.has(type);
@@ -141,9 +183,12 @@ export function foldDailyStats(
       }
     }
     const picked = collapseRollupRowsBySource(cells, type, priorityJson);
-    const perDay = new Map<string, number>();
+    const perDay = new Map<string, DayCell>();
     for (const cell of picked) {
-      perDay.set(dateOnlyKey(cell.bucketStart), cell.value);
+      perDay.set(dateOnlyKey(cell.bucketStart), {
+        value: cell.value,
+        count: cell.count,
+      });
     }
     out.set(type, perDay);
   }
diff --git a/src/lib/day/wire-schemas.ts b/src/lib/day/wire-schemas.ts
index 1a535c4b1..6cfa4e118 100644
--- a/src/lib/day/wire-schemas.ts
+++ b/src/lib/day/wire-schemas.ts
@@ -22,6 +22,7 @@ import {
   LIFE_EVENT_NOTE_MAX,
   LIFE_EVENT_PRECISIONS,
   LIFE_EVENT_TITLE_MAX,
+  TIMELINE_BUCKETS,
   TIMELINE_ITEM_KINDS,
   TIMELINE_LANE_KEYS,
   TIMELINE_READINESS_KEYS,
@@ -267,7 +268,7 @@ export const timelineQuerySchema = z.object({
     .string()
     .optional()
     .describe(
-      "Comma-separated value series to include, as `MeasurementType` names or `MOOD`.",
+      "Comma-separated value series to include, as `MeasurementType` names or `MOOD`, at most 6. More, or an unknown name, answers 422.",
     ),
 });
 
@@ -304,8 +305,27 @@ const timelineSeries = z
   .object({
     key: z.string().describe("A `MeasurementType` or `MOOD`."),
     unit: z.string().nullable(),
-    granularity: z.enum(["month", "week", "day"]),
-    points: z.array(z.object({ t: dateKeySchema, mean: z.number() })),
+    points: z
+      .array(
+        z.object({
+          t: dateKeySchema.describe("The bucket's first day."),
+          mean: z
+            .number()
+            .describe(
+              "The mean of the bucket's day values, each day weighing one (a cumulative type's days are its day totals, a pulse-like type's its hourly-mean day value).",
+            ),
+          count: z
+            .number()
+            .int()
+            .min(1)
+            .describe(
+              "Readings behind the mean. Clients draw a point from fewer than three as thin.",
+            ),
+        }),
+      )
+      .describe(
+        "Buckets with at least one reading, oldest first. A bucket without one is absent, never interpolated.",
+      ),
   })
   .meta({ id: "TimelineSeries" });
 
@@ -328,6 +348,11 @@ export const timelineResponseSchema = z
     standing: z
       .array(timelineStanding)
       .describe("Things without a start: shown once as standing."),
+    bucket: z
+      .enum(TIMELINE_BUCKETS)
+      .describe(
+        "The span every series point averages: `quarter` for a multi-year `all`, `month` for `year` and an `all` under two years, `week` for `quarter`.",
+      ),
     series: z.array(timelineSeries),
     notable: z.array(
       z.object({ date: dateKeySchema, kind: z.enum(DAY_NOTABLE_KINDS) }),
diff --git a/src/lib/timeline/__tests__/series.test.ts b/src/lib/timeline/__tests__/series.test.ts
index 0e0506328..b9aa6d4c4 100644
--- a/src/lib/timeline/__tests__/series.test.ts
+++ b/src/lib/timeline/__tests__/series.test.ts
@@ -3,9 +3,9 @@
  */
 import { describe, expect, it } from "vitest";
 
-import { overlaps, parseSeriesKeys } from "../load-timeline";
+import { overlaps, parseSeriesKeys, timelineBucket } from "../load-timeline";
 import { coveredWeeks, READINESS_WEEKS } from "../readiness";
-import { bucketKey, foldDays } from "../series";
+import { bucketKey, foldDays, foldMonths } from "../series";
 
 describe("buckets", () => {
   it("names a bucket by its first local day, weeks from Monday", () => {
@@ -13,23 +13,65 @@ describe("buckets", () => {
     // 2026-03-29 is a Sunday; its week began on Monday the 23rd.
     expect(bucketKey("2026-03-29", "week")).toBe("2026-03-23");
     expect(bucketKey("2026-03-23", "week")).toBe("2026-03-23");
-    expect(bucketKey("2026-03-29", "day")).toBe("2026-03-29");
+    expect(bucketKey("2026-03-29", "quarter")).toBe("2026-01-01");
+    expect(bucketKey("2026-04-01", "quarter")).toBe("2026-04-01");
+    expect(bucketKey("2026-12-31", "quarter")).toBe("2026-10-01");
   });
 
-  it("weighs each day once in a bucket", () => {
+  it("weighs each day once in a bucket and counts the readings behind it", () => {
     const points = foldDays(
       new Map([
-        ["2026-03-01", 100],
-        ["2026-03-02", 110],
-        ["2026-04-01", 90],
+        ["2026-03-01", { value: 100, count: 3 }],
+        ["2026-03-02", { value: 110, count: 1 }],
+        ["2026-04-01", { value: 90, count: 1 }],
       ]),
       "month",
     );
     expect(points).toEqual([
-      { t: "2026-03-01", mean: 105 },
-      { t: "2026-04-01", mean: 90 },
+      { t: "2026-03-01", mean: 105, count: 4 },
+      { t: "2026-04-01", mean: 90, count: 1 },
     ]);
   });
+
+  it("leaves a bucket without a reading out rather than filling it", () => {
+    const points = foldDays(
+      new Map([
+        ["2026-02-10", { value: 120, count: 2 }],
+        ["2026-06-10", { value: 130, count: 2 }],
+        ["2026-08-10", { value: 125, count: 1 }],
+      ]),
+      "month",
+    );
+    expect(points.map((p) => p.t)).toEqual([
+      "2026-02-01",
+      "2026-06-01",
+      "2026-08-01",
+    ]);
+  });
+
+  it("folds rolled-up months into quarters, each month weighing its days", () => {
+    expect(
+      foldMonths(
+        [
+          { t: "2026-01-01", mean: 100, days: 30, count: 30 },
+          { t: "2026-02-01", mean: 130, days: 10, count: 12 },
+          { t: "2026-04-01", mean: 90, days: 2, count: 2 },
+        ],
+        "quarter",
+      ),
+    ).toEqual([
+      // (100·30 + 130·10) / 40, not the plain mean of the two months (115).
+      { t: "2026-01-01", mean: 107.5, count: 42 },
+      { t: "2026-04-01", mean: 90, count: 2 },
+    ]);
+  });
+
+  it("picks the bucket by zoom, quarters only for a long `all`", () => {
+    expect(timelineBucket("quarter", "2026-01-01", "2026-03-31")).toBe("week");
+    expect(timelineBucket("year", "2025-04-01", "2026-03-31")).toBe("month");
+    expect(timelineBucket("all", "2025-01-01", "2026-03-31")).toBe("month");
+    expect(timelineBucket("all", "2019-01-01", "2026-03-31")).toBe("quarter");
+  });
 });
 
 describe("window", () => {
@@ -55,6 +97,19 @@ describe("window", () => {
     expect(parseSeriesKeys("WEIGHT,MOOD,WEIGHT")).toEqual(["WEIGHT", "MOOD"]);
     expect(parseSeriesKeys("WEIGHT,DROP TABLE")).toBeNull();
   });
+
+  it("accepts up to six series and refuses a seventh", () => {
+    const six = [
+      "BLOOD_PRESSURE_SYS",
+      "BLOOD_PRESSURE_DIA",
+      "WEIGHT",
+      "PULSE",
+      "MOOD",
+      "BODY_FAT",
+    ];
+    expect(parseSeriesKeys(six.join(","))).toEqual(six);
+    expect(parseSeriesKeys([...six, "BLOOD_GLUCOSE"].join(","))).toBeNull();
+  });
 });
 
 describe("readiness weeks", () => {
diff --git a/src/lib/timeline/load-timeline.ts b/src/lib/timeline/load-timeline.ts
index 0b0691b54..bc97b723d 100644
--- a/src/lib/timeline/load-timeline.ts
+++ b/src/lib/timeline/load-timeline.ts
@@ -18,6 +18,8 @@ import type { MeasurementType } from "@/generated/prisma/enums";
 import {
   DAY_NOTABLE_MAX_SPAN_DAYS,
   TIMELINE_LANE_KEYS,
+  TIMELINE_MAX_SERIES,
+  type TimelineBucket,
   type TimelineItem,
   type TimelineQuery,
   type TimelineResponse,
@@ -33,12 +35,8 @@ import {
   type StandingItem,
   type TimelineAccess,
 } from "@/lib/timeline/lanes";
-import {
-  loadTimelineSeries,
-  MOOD_SERIES_KEY,
-  type SeriesGranularity,
-} from "@/lib/timeline/series";
-import { shiftDateKey, userDayKey } from "@/lib/tz/format";
+import { loadTimelineSeries, MOOD_SERIES_KEY } from "@/lib/timeline/series";
+import { daysBetweenDateKeys, shiftDateKey, userDayKey } from "@/lib/tz/format";
 
 /** The series sent when the request names none. */
 export const DEFAULT_TIMELINE_SERIES: readonly MeasurementType[] = [
@@ -47,11 +45,27 @@ export const DEFAULT_TIMELINE_SERIES: readonly MeasurementType[] = [
   "WEIGHT",
 ];
 
-const ZOOM_GRANULARITY: Readonly<Record<TimelineZoom, SeriesGranularity>> = {
-  all: "month",
-  year: "week",
-  quarter: "day",
-};
+/** An `all` window longer than this many days averages quarters. */
+const QUARTER_BUCKET_MIN_DAYS = 2 * 365;
+
+/**
+ * The span one series point averages. A point per day or per week over
+ * years drew a line of fragments wherever readings came in bursts, so the
+ * bucket grows with the window: weeks in three months (thirteen points),
+ * months in a year (twelve), quarters once `all` reaches back more than two
+ * years. An `all` window shorter than that keeps months, so a young record
+ * does not shrink to a handful of quarters.
+ */
+export function timelineBucket(
+  zoom: TimelineZoom,
+  from: string,
+  to: string,
+): TimelineBucket {
+  if (zoom === "quarter") return "week";
+  if (zoom === "year") return "month";
+  const days = daysBetweenDateKeys(from, to);
+  return days > QUARTER_BUCKET_MIN_DAYS ? "quarter" : "month";
+}
 
 const ZOOM_DAYS: Readonly<Record<Exclude<TimelineZoom, "all">, number>> = {
   year: 365,
@@ -64,7 +78,8 @@ const MEASUREMENT_TYPE_SET: ReadonlySet<string> = new Set(
 
 /**
  * The series keys a `values` parameter names, or null when one is not a
- * series this API knows (the route answers 422).
+ * series this API knows or there are more than {@link TIMELINE_MAX_SERIES}
+ * (the route answers 422).
  */
 export function parseSeriesKeys(raw: string | undefined): string[] | null {
   if (raw === undefined) return [...DEFAULT_TIMELINE_SERIES];
@@ -76,7 +91,7 @@ export function parseSeriesKeys(raw: string | undefined): string[] | null {
         .filter((k) => k.length > 0),
     ),
   ];
-  if (keys.length > 8) return null;
+  if (keys.length > TIMELINE_MAX_SERIES) return null;
   for (const key of keys) {
     if (key !== MOOD_SERIES_KEY && !MEASUREMENT_TYPE_SET.has(key)) return null;
   }
@@ -158,6 +173,7 @@ export async function loadTimeline(args: {
     shiftDateKey(to, -(DAY_NOTABLE_MAX_SPAN_DAYS - 1)) > from
       ? shiftDateKey(to, -(DAY_NOTABLE_MAX_SPAN_DAYS - 1))
       : from;
+  const bucket = timelineBucket(query.zoom, from, to);
   const [series, notable] = await Promise.all([
     loadTimelineSeries({
       userId: recordId,
@@ -165,7 +181,7 @@ export async function loadTimeline(args: {
       from,
       to,
       tz,
-      granularity: ZOOM_GRANULARITY[query.zoom],
+      bucket,
       priorityJson,
       now,
     }),
@@ -187,6 +203,7 @@ export async function loadTimeline(args: {
     range: { from, to, dataFrom },
     lanes,
     standing,
+    bucket,
     series,
     notable: notable.map(({ date, kind }) => ({ date, kind })),
   };
diff --git a/src/lib/timeline/series.ts b/src/lib/timeline/series.ts
index aa51c1024..9dd6a56b7 100644
--- a/src/lib/timeline/series.ts
+++ b/src/lib/timeline/series.ts
@@ -1,76 +1,137 @@
 /**
- * Value series for the timeline: monthly, weekly or daily means beside the
- * lanes (v1.42, #613).
+ * Value series for the timeline: quarterly, monthly or weekly means beside
+ * the lanes (v1.42, #613).
  *
- * Three ways in, by zoom:
+ * The bucket comes with the zoom (`timelineBucket` in `load-timeline.ts`),
+ * and the source with the window's length:
  *
- *   - **all (month)**: the months of the last five years come from the rollup
- *     tier, MONTH buckets folded from the canonical-source DAY rows. Those
- *     buckets cut at UTC midnight, so a reading in the first or last hours of
- *     a month can sit in the neighbouring month. For a monthly mean over
+ *   - **longer than a year** (`all`): the last five years come from the
+ *     rollup tier, MONTH buckets folded from the canonical-source DAY rows
+ *     and, for quarters, folded once more here, each month weighing its days.
+ *     Those buckets cut at UTC midnight, so a reading in the first or last
+ *     hours of a month can sit in the neighbouring month. For a mean over
  *     dozens of days that is a rounding matter, and it is accepted here; the
  *     day view, where it is not, never reads the rollups. The rollups fold
- *     only five years (`ROLLUP_FOLD_WINDOW_MS`); the months before that come
+ *     only five years (`ROLLUP_FOLD_WINDOW_MS`); the buckets before that come
  *     from one live query over the record's own local days, cached per record
  *     in the analytics bucket, which every measurement write marks stale.
- *   - **year (week)** and **quarter (day)**: live over the record's local
- *     days, a year at most.
+ *   - **a year or less** (`year`, `quarter`, a short `all`): live over the
+ *     record's local days.
  *
  * Every value is the mean of the bucket's days, each day weighing one (a
- * cumulative type's days are its day totals), matching the charts. A series
+ * cumulative type's days are its day totals, a pulse-like type's day is the
+ * mean of its hourly means, `daily-stats.ts`), matching the charts. Each
+ * point carries the readings behind it, so the chart can mark a mean that
+ * rests on one or two. A bucket without a reading is not sent, and a series
  * without a point is not sent. `MOOD` is the mood score, read from the
- * entries' own dates.
+ * entries' own dates; its count is the entries.
  */
 import type { MeasurementType } from "@/generated/prisma/enums";
 
 import { caches, cached } from "@/lib/cache/server-cache";
-import { readLocalDailyStats } from "@/lib/day/daily-stats";
-import type { TimelineSeries } from "@/lib/day/contract";
+import { readLocalDailyCells, type DayCell } from "@/lib/day/daily-stats";
+import type { TimelineBucket, TimelineSeries } from "@/lib/day/contract";
 import { prisma } from "@/lib/db";
 import { isCumulativeDaySumType } from "@/lib/measurements/cumulative-day-sum";
 import { readCanonicalRollupBuckets } from "@/lib/rollups/measurement-read";
 import { ROLLUP_FOLD_WINDOW_MS } from "@/lib/rollups/measurement-rollups";
 import { dateOnlyKey } from "@/lib/tz/date-only";
-import { shiftDateKey, userDayKey, weekdayOfDateKey } from "@/lib/tz/format";
+import {
+  daysBetweenDateKeys,
+  shiftDateKey,
+  userDayKey,
+  weekdayOfDateKey,
+} from "@/lib/tz/format";
 import { startOfLocalDayKey } from "@/lib/tz/local-day";
 
-export type SeriesGranularity = TimelineSeries["granularity"];
+export type SeriesPoint = TimelineSeries["points"][number];
 
 /**
- * How long the months before the fold window stay cached. They change only
+ * How long the buckets before the fold window stay cached. They change only
  * when an old reading is imported or deleted, and either write marks the
  * entry stale at once.
  */
 const PRE_FOLD_TTL_MS = 6 * 3_600_000;
 
+/** Windows longer than this read the rollup tier rather than live days. */
+const LIVE_WINDOW_MAX_DAYS = 366;
+
 /** The series key for the mood score. */
 export const MOOD_SERIES_KEY = "MOOD";
 
 /** The bucket a local day belongs to, named by its first day. */
-export function bucketKey(day: string, granularity: SeriesGranularity): string {
-  if (granularity === "day") return day;
-  if (granularity === "month") return `${day.slice(0, 7)}-01`;
+export function bucketKey(day: string, bucket: TimelineBucket): string {
+  if (bucket === "month") return `${day.slice(0, 7)}-01`;
+  if (bucket === "quarter") {
+    const month = Number(day.slice(5, 7));
+    const first = month - ((month - 1) % 3);
+    return `${day.slice(0, 4)}-${String(first).padStart(2, "0")}-01`;
+  }
   // Weeks start on Monday.
   const offset = (weekdayOfDateKey(day) + 6) % 7;
   return shiftDateKey(day, -offset);
 }
 
-/** Fold a day series into buckets: the mean of the bucket's days. */
+const round2 = (v: number) => Math.round(v * 100) / 100;
+
+/**
+ * Fold a day series into buckets: the mean of the bucket's days, and the
+ * readings behind them.
+ */
 export function foldDays(
-  days: ReadonlyMap<string, number>,
-  granularity: SeriesGranularity,
-): Array<{ t: string; mean: number }> {
-  const buckets = new Map<string, { sum: number; n: number }>();
-  for (const [day, value] of days) {
-    const key = bucketKey(day, granularity);
-    const slot = buckets.get(key) ?? { sum: 0, n: 0 };
-    slot.sum += value;
+  days: ReadonlyMap<string, DayCell>,
+  bucket: TimelineBucket,
+): SeriesPoint[] {
+  const buckets = new Map<string, { sum: number; n: number; count: number }>();
+  for (const [day, cell] of days) {
+    const key = bucketKey(day, bucket);
+    const slot = buckets.get(key) ?? { sum: 0, n: 0, count: 0 };
+    slot.sum += cell.value;
     slot.n += 1;
+    slot.count += cell.count;
     buckets.set(key, slot);
   }
   return [...buckets]
     .sort((a, b) => a[0].localeCompare(b[0]))
-    .map(([t, { sum, n }]) => ({ t, mean: Math.round((sum / n) * 100) / 100 }));
+    .map(([t, { sum, n, count }]) => ({ t, mean: round2(sum / n), count }));
+}
+
+/** A month from the rollup tier: its day mean, its days, its readings. */
+export interface RolledMonth {
+  t: string;
+  mean: number;
+  days: number;
+  count: number;
+}
+
+/**
+ * Fold rolled-up months into `bucket`: each month weighs its days, so a
+ * quarter's mean is the mean of its days exactly as a live fold makes it.
+ */
+export function foldMonths(
+  months: readonly RolledMonth[],
+  bucket: Exclude<TimelineBucket, "week">,
+): SeriesPoint[] {
+  const buckets = new Map<
+    string,
+    { sum: number; days: number; count: number }
+  >();
+  for (const m of months) {
+    const key = bucketKey(m.t, bucket);
+    const slot = buckets.get(key) ?? { sum: 0, days: 0, count: 0 };
+    slot.sum += m.mean * m.days;
+    slot.days += m.days;
+    slot.count += m.count;
+    buckets.set(key, slot);
+  }
+  return [...buckets]
+    .sort((a, b) => a[0].localeCompare(b[0]))
+    .filter(([, b]) => b.days > 0 && b.count > 0)
+    .map(([t, { sum, days, count }]) => ({
+      t,
+      mean: round2(sum / days),
+      count,
+    }));
 }
 
 async function latestUnit(
@@ -85,24 +146,25 @@ async function latestUnit(
   return row?.unit ?? null;
 }
 
-/** Months older than the rollup fold window, live and cached. */
-async function preFoldMonths(args: {
+/** Buckets older than the rollup fold window, live and cached. */
+async function preFoldBuckets(args: {
   userId: string;
   type: MeasurementType;
   tz: string;
   from: Date;
   boundary: Date;
+  bucket: TimelineBucket;
   priorityJson: unknown;
-}): Promise<Array<{ t: string; mean: number }>> {
+}): Promise<SeriesPoint[]> {
   if (args.from >= args.boundary) return [];
   // Keyed under the record's `${userId}|` prefix: every measurement write
   // marks it stale with the rest of the analytics bucket.
-  const key = `${args.userId}|timeline-pre-fold|${args.type}|${args.tz}|${dateOnlyKey(args.from)}|${dateOnlyKey(args.boundary)}`;
+  const key = `${args.userId}|timeline-pre-fold|${args.type}|${args.bucket}|${args.tz}|${dateOnlyKey(args.from)}|${dateOnlyKey(args.boundary)}`;
   return cached(
     caches.analytics,
     key,
     async () => {
-      const stats = await readLocalDailyStats({
+      const cells = await readLocalDailyCells({
         userId: args.userId,
         types: [args.type],
         from: args.from,
@@ -110,20 +172,20 @@ async function preFoldMonths(args: {
         tz: args.tz,
         priorityJson: args.priorityJson,
       });
-      return foldDays(stats.get(args.type) ?? new Map(), "month");
+      return foldDays(cells.get(args.type) ?? new Map(), args.bucket);
     },
     undefined,
     PRE_FOLD_TTL_MS,
-  ) as Promise<Array<{ t: string; mean: number }>>;
+  ) as Promise<SeriesPoint[]>;
 }
 
-async function monthlyFromRollups(args: {
+async function monthsFromRollups(args: {
   userId: string;
   type: MeasurementType;
   from: Date;
   to: Date;
   priorityJson: unknown;
-}): Promise<Array<{ t: string; mean: number }>> {
+}): Promise<RolledMonth[]> {
   const rows = await readCanonicalRollupBuckets({
     userId: args.userId,
     type: args.type,
@@ -142,59 +204,67 @@ async function monthlyFromRollups(args: {
         : r.mean;
     return {
       t: `${dateOnlyKey(r.bucketStart).slice(0, 7)}-01`,
-      mean: Math.round(value * 100) / 100,
+      mean: value,
+      days: Math.max(1, r.days),
+      count: r.count,
     };
   });
 }
 
+/** Whether `[from, to]` is long enough to read the rollup tier. */
+function readsRollups(from: string, to: string): boolean {
+  const days = daysBetweenDateKeys(from, to);
+  return days >= LIVE_WINDOW_MAX_DAYS;
+}
+
 async function measurementSeries(args: {
   userId: string;
   type: MeasurementType;
   from: string;
   to: string;
   tz: string;
-  granularity: SeriesGranularity;
+  bucket: TimelineBucket;
   priorityJson: unknown;
   now: Date;
 }): Promise<TimelineSeries | null> {
   const start = startOfLocalDayKey(args.from, args.tz);
   const end = startOfLocalDayKey(shiftDateKey(args.to, 1), args.tz);
-  let points: Array<{ t: string; mean: number }>;
-  if (args.granularity === "month") {
-    // The fold boundary rounded up to a month start, so no month is half
+  let points: SeriesPoint[];
+  if (args.bucket !== "week" && readsRollups(args.from, args.to)) {
+    const bucket = args.bucket;
+    // The fold boundary rounded up to a bucket start, so no bucket is half
     // live and half rolled up.
     const raw = new Date(args.now.getTime() - ROLLUP_FOLD_WINDOW_MS);
     const boundaryKey = bucketKey(
-      shiftDateKey(userDayKey(raw, args.tz), 31),
-      "month",
+      shiftDateKey(userDayKey(raw, args.tz), bucket === "quarter" ? 92 : 31),
+      bucket,
     );
     const boundary = startOfLocalDayKey(boundaryKey, args.tz);
     const [older, recent] = await Promise.all([
-      preFoldMonths({
+      preFoldBuckets({
         userId: args.userId,
         type: args.type,
         tz: args.tz,
         from: start,
         boundary: boundary < end ? boundary : end,
+        bucket,
         priorityJson: args.priorityJson,
       }),
       end > boundary
-        ? monthlyFromRollups({
+        ? monthsFromRollups({
             userId: args.userId,
             type: args.type,
             from: boundary > start ? boundary : start,
             to: end,
             priorityJson: args.priorityJson,
-          })
+          }).then((months) => foldMonths(months, bucket))
         : Promise.resolve([]),
     ]);
-    const merged = new Map<string, number>();
-    for (const p of [...older, ...recent]) merged.set(p.t, p.mean);
-    points = [...merged]
-      .sort((a, b) => a[0].localeCompare(b[0]))
-      .map(([t, mean]) => ({ t, mean }));
+    const merged = new Map<string, SeriesPoint>();
+    for (const p of [...older, ...recent]) merged.set(p.t, p);
+    points = [...merged.values()].sort((a, b) => a.t.localeCompare(b.t));
   } else {
-    const stats = await readLocalDailyStats({
+    const cells = await readLocalDailyCells({
       userId: args.userId,
       types: [args.type],
       from: start,
@@ -202,13 +272,12 @@ async function measurementSeries(args: {
       tz: args.tz,
       priorityJson: args.priorityJson,
     });
-    points = foldDays(stats.get(args.type) ?? new Map(), args.granularity);
+    points = foldDays(cells.get(args.type) ?? new Map(), args.bucket);
   }
   if (points.length === 0) return null;
   return {
     key: args.type,
     unit: await latestUnit(args.userId, args.type),
-    granularity: args.granularity,
     points,
   };
 }
@@ -217,7 +286,7 @@ async function moodSeries(args: {
   userId: string;
   from: string;
   to: string;
-  granularity: SeriesGranularity;
+  bucket: TimelineBucket;
 }): Promise<TimelineSeries | null> {
   const rows = await prisma.moodEntry.groupBy({
     by: ["date"],
@@ -227,17 +296,19 @@ async function moodSeries(args: {
       date: { gte: args.from, lte: args.to },
     },
     _avg: { score: true },
+    _count: { score: true },
   });
-  const days = new Map<string, number>();
+  const days = new Map<string, DayCell>();
   for (const row of rows) {
-    if (row._avg.score !== null) days.set(row.date, row._avg.score);
+    if (row._avg.score !== null && row._count.score > 0) {
+      days.set(row.date, { value: row._avg.score, count: row._count.score });
+    }
   }
-  const points = foldDays(days, args.granularity);
+  const points = foldDays(days, args.bucket);
   if (points.length === 0) return null;
   return {
     key: MOOD_SERIES_KEY,
     unit: null,
-    granularity: args.granularity,
     points,
   };
 }
@@ -248,7 +319,7 @@ export async function loadTimelineSeries(args: {
   from: string;
   to: string;
   tz: string;
-  granularity: SeriesGranularity;
+  bucket: TimelineBucket;
   priorityJson: unknown;
   now?: Date;
 }): Promise<TimelineSeries[]> {
diff --git a/tests/integration/day-view.test.ts b/tests/integration/day-view.test.ts
index 1633cd3cd..35df59adf 100644
--- a/tests/integration/day-view.test.ts
+++ b/tests/integration/day-view.test.ts
@@ -837,6 +837,125 @@ describe("timeline", () => {
     expect(byKey.values.status).toBe("empty");
     expect(inventory.verdict).toBe("thin");
   });
+
+  it("averages value series per bucket, counts the readings and leaves gaps out", async () => {
+    const db = getPrismaClient();
+    const owner = await makeUser("tl-values", { timeline: true });
+    const reading = (at: string, value: number) => ({
+      userId: owner.id,
+      type: "BLOOD_PRESSURE_SYS" as const,
+      value,
+      unit: "mmHg",
+      measuredAt: new Date(at),
+    });
+    await db.measurement.createMany({
+      data: [
+        // Two readings on one January day, one on another: three readings,
+        // two days, each day weighing one.
+        reading("2026-01-10T07:00:00.000Z", 120),
+        reading("2026-01-10T19:00:00.000Z", 130),
+        reading("2026-01-20T07:00:00.000Z", 135),
+        // Nothing in February.
+        reading("2026-03-05T07:00:00.000Z", 128),
+      ],
+    });
+    await signIn(owner.id);
+
+    const { GET } = await import("@/app/api/timeline/route");
+    const timeline = await json<import("@/lib/day/contract").TimelineResponse>(
+      await call(
+        GET as Handler,
+        "GET",
+        "/api/timeline?zoom=year&to=2026-03-31&values=BLOOD_PRESSURE_SYS",
+      ),
+    );
+    expect(timeline.bucket).toBe("month");
+    expect(timeline.series).toEqual([
+      {
+        key: "BLOOD_PRESSURE_SYS",
+        unit: "mmHg",
+        points: [
+          { t: "2026-01-01", mean: 130, count: 3 },
+          { t: "2026-03-01", mean: 128, count: 1 },
+        ],
+      },
+    ]);
+
+    const weekly = await json<import("@/lib/day/contract").TimelineResponse>(
+      await call(
+        GET as Handler,
+        "GET",
+        "/api/timeline?zoom=quarter&to=2026-03-31&values=BLOOD_PRESSURE_SYS",
+      ),
+    );
+    expect(weekly.bucket).toBe("week");
+    expect(weekly.series[0].points.map((p) => [p.t, p.count])).toEqual([
+      ["2026-01-05", 2],
+      ["2026-01-19", 1],
+      ["2026-03-02", 1],
+    ]);
+
+    const six =
+      "BLOOD_PRESSURE_SYS,BLOOD_PRESSURE_DIA,WEIGHT,PULSE,BODY_FAT,MOOD";
+    expect(
+      (await call(GET as Handler, "GET", `/api/timeline?values=${six}`)).status,
+    ).toBe(200);
+    expect(
+      (
+        await call(
+          GET as Handler,
+          "GET",
+          `/api/timeline?values=${six},BLOOD_GLUCOSE`,
+        )
+      ).status,
+    ).toBe(422);
+  });
+
+  it("averages quarters over the years, live before the fold window and rolled up after it", async () => {
+    const db = getPrismaClient();
+    const owner = await makeUser("tl-quarters", { timeline: true });
+    const reading = (at: string, value: number) => ({
+      userId: owner.id,
+      type: "WEIGHT" as const,
+      value,
+      unit: "kg",
+      measuredAt: new Date(at),
+    });
+    await db.measurement.createMany({
+      data: [
+        // Before the five-year fold window: read live.
+        reading("2018-02-10T07:00:00.000Z", 90),
+        reading("2018-03-10T07:00:00.000Z", 88),
+        // Inside it: read from the rollups. January has three days,
+        // February one; the quarter's mean weighs the four days alike.
+        reading("2024-01-10T07:00:00.000Z", 80),
+        reading("2024-01-11T07:00:00.000Z", 80),
+        reading("2024-01-12T07:00:00.000Z", 80),
+        reading("2024-02-10T07:00:00.000Z", 84),
+        // Nothing from April 2024 to March 2025.
+        reading("2025-05-10T07:00:00.000Z", 79),
+      ],
+    });
+    const { recomputeUserRollups } =
+      await import("@/lib/rollups/measurement-rollups");
+    await recomputeUserRollups(owner.id, {
+      types: ["WEIGHT"],
+      from: new Date("2018-01-01T00:00:00.000Z"),
+      to: new Date(),
+    });
+    await signIn(owner.id);
+
+    const { GET } = await import("@/app/api/timeline/route");
+    const timeline = await json<import("@/lib/day/contract").TimelineResponse>(
+      await call(GET as Handler, "GET", "/api/timeline?zoom=all&values=WEIGHT"),
+    );
+    expect(timeline.bucket).toBe("quarter");
+    expect(timeline.series[0].points).toEqual([
+      { t: "2018-01-01", mean: 89, count: 2 },
+      { t: "2024-01-01", mean: 81, count: 4 },
+      { t: "2025-04-01", mean: 79, count: 1 },
+    ]);
+  });
 });
 
 describe("performance smoke", () => {