AFAIK, there is no way to do that. But you could set up a custom drilldown such that when you click on a bar, another panel displays that shows the details.
You can see examples of this if you download the Splunk 6 Dashboards Examples app, which will also show you the code.
Or you can just look it up in the Dashboards and Visualizations manual (which I think is more work and less fun.) I have the app installed on my laptop, just so I can use it to figure out how to do cool stuff like this (and much more).