We don't currently have legends implemented in the bubble chart - the taxonomic/category annotations pop up when you click on a specific bubble (since the bubble chart was meant to facilitate more fine-scale exploration of the data). The colors doesn't necessarily correspond between the bar chart and bubble chart, so I would caution against pulling the legend over from a different visualization.
I will add the request for bubble chart legends to our "to do" list for future development priorities. Thanks for raising the issue.