Hi Andy,
This may not be what you want, but maybe it will inspire someone to offer a better idea.
To build the legend:
- I built a Graph Builder graph with a size role using the same column as the bubble plot.
- I shrunk the graph and removed the border and title.
- I deleted the Graph Builder title, which removed the Graph Builder Outline box.
Then I combined windows to create a dashboard.
If this works for you, please let me know if you need more details steps.
~John