When trying to create an irregularly spaced intervals in a colorbar, generated in Axes_Grid1.ImageGrid, I noticed a problem with the color bar tickmark labels. In essence when using ImageGrid you cannot directly create an uniformly spaced colorbar with irregular levels, however, you can when not using ImageGrid. Below is a self-contained sample script that demonstrates this problem.

https://gist.github.com/1086530

This occurs on both Linux (MPL from git) and OS X (MPL V1.0.1)