Add a legend with summary stats for stopbuffers

This adds a simple summary-stats legend for stopbuffers. Like so:

![screen shot 2014-12-23 at 11 59 18 am 2](https://cloud.githubusercontent.com/assets/1977405/5540631/bd406720-8a9b-11e4-992e-8a79a817b686.png)

Is this used anywhere? If not, remove.