Hi Suzanne,
Posting questions about an issue in the forum first is a good idea. When you are sure you have found a bug, then posting on the GitHub issues list is fine.
For your issue, this is a known problem area, which should be better documented at least. If your chart container element is positioned, or scrollable, or has padding, then the sensitivity of where the mouse appears to be will be offset from where it actually is. I'm not sure we can deal with this properly inside of the charts code, though it would seem we should be able to. It has been a long-standing issue that we haven't tried to deal with, mostly because there is a workaround.
I also set the height option, and the chartArea height and bottom to enable the full display of the tick labels.