FileMaker charting seems to have been neglected for far too long. Hopefully when a change is made they will have a whole new charting system that works. I try to avoid FM charts as much as possible. They are not sexy, have issues, and take up far too much real estate. I do not see them fixing it as it seems they have come to see the shortcomings as "expected behavior".
Thank you for your post and screen shots.
Our Development and Testing departments are aware of this overlapping of values. I have attached your post and screen shots to the original report. When more information becomes available, I will post again.