We were able to observe this issue and logged it in our database for investigation and for a fix. Once, the issue is resolved or we have some other news for you, we will let you know asap.
This issue has been logged as
CELLSJAVA-42561 - X-Axis Scale is incorrect in PNG output of Excel Chart
This is to inform you that we have fixed your issue CELLSJAVA-42561 now. We will soon provide the fix after performing QA and including other enhancements and fixes.