Hiding a marker takes it out of the labs catalogue and the entry
pickers, but the timeline still offered it among the lab values.
The timeline now offers only markers that are not hidden. A hidden
marker that was chosen before keeps drawing its line and stays in the
menu as a ticked entry, named from that line, so it can be unticked,
the way the entry form keeps a hidden marker that is already selected.
Once unticked it is no longer offered.