This is not possible with the current version of FileMaker Charting. Both sets of values will use the same scale.
Thank you for your post, and I apologize for the late reply.
The answer provided by "PhilModJunk" is correct (Thank you!).
I encourage you to enter this as a suggestion into our Feature Requests web form at:
The entries into this web form are monitored and read by members of Product Management and Development, where they are then discussed and considered for future releases. Although I could copy your post and paste it into the web form, there are a couple of questions asked that only you can answer.