Hi Andrew,
Apologies for the late reply. I've been holding off on responding until I could check with the Dev team.
> As the screenshots below show, I can do this on individual charts but I'd love to combine then into one.
What you're asking for is not possible at this time. Please submit an enhancement on the ArcGIS Ideas site - although, I think you'll need to be specific on the requirement.
Sorry again for the delayed response.
Hope this helps,