Hi Kent,
I couldn't find any options for doing this, so I dug into the Data Graphics elements and did a simple bit of ShapeSheet maneuvering. If you're not familiar with the ShapeSheet, and the internal structure of Visio diagrams, this is decidedly NOT simple, so don't feel bad!
If you are a Visio Nerd like a lot of us on this forum, then enjoy looking inside the Text callout hide blank data graphic item that I have edited.
There are more notes in the attached Visio file...