You are right and this is a known bug, also listed in changes.txt already.
I assume the bug is on jpgraph side, as jpgraph3.5 does not have this problem.
Sadly jpgraph 3.5 suffers from even more problems. To me it seems like the 3.5 branch is not fully implemented yet.
As soon as i can switch to 3.5 safely i will do so. Until that we have to live with that
Thank you for the report