I can think of two ways you could accomplish this: tooltip and annotation.
You could use a tooltip -- modify the existing tooltip to include the text comment you wish to add about the labels. If the comment would be different per label, the tooltip can be created to update dynamically, but that takes some effort. This method conceals the comment until the user hovers over the bar.
You could also annotate the mark. To do this, right click on the bar containing the label you wish to comment and choose Annotate - Mark. You can now type in a comment about that label and position it appropriately. This method makes the comment always visible.
See the attached workbook for an example of each.
Hope this helps,
Book6.twbx.zip 1.1 MB
Thanks Shawn, I'm assuming this is the same logic that Tableau uses for the multiple labels option in Tableau 8?
This was very helpful. I think I'm going to use both until 8 is released.
Actually in 8 it's a lot easier, you just drop as many measures as you want on the label shelf and they appear on the marks, all nicely formatted (if memory serves). None of the rounding-stringing-lookupy stuff.