You calculate local coordinates there based on evt.pageX/evt.pageY and canvas page bounds. Element page bounds variable gets updated on "resize" page event(and manual "resize" calls), but page layout may be changed all the time(some data is loaded and element above chart became bigger for example).
Easiest fix is to use evt.offsetX/evt.offsetY, but this also highlight problem with "bounds" variable, better not to use it if possible since there is no performant tracker of element position.