when manually assigning labels, a click selects a the first curve in the set of curves with a bounding area that contains the click location. First is determined by the z order which is the order in which the curves were added to the scene, I think.

For two adjacent whiskers this can become irritating because the draw order isn't really related to anything. One whisker may be drawn on top of another in one frame, but behind it the next. Annotating requires infering the draw order by trying to click on things, changing to the correct whisker i (the last one in the draw order), clicking through to that curve, and then repeating for all the shallower whiskers.