Each subplot carries its own copy of the shared time cursor, but the drag handler always
read the first copy's value, so dragging FX or FY re-read FN's unchanged position and
returned without moving the others. The handler now uses the line the signal names.
Verified by dragging all three cursors in the offscreen UI test and by the full studio
suite (569 tests, 0 failed).
ykcheungab
d89535236b