Think about how to save axes information when making multiple figures.

[scfleming]

Currently the software will generate n plots, doing the various calculations n times. This is because I couldn't figure out a way to get multiple matplotlib figure instances to use the same axes instances, and from what I read it's not terribly easy to do. I think I would need to completely refactor plotspec to operate on axes objects instead of figure objects, so pushing this to a later build. It could improve run time by a linear amount though.

[scfleming]

This is somewhat minimized now that a lot of the plot metrics code is moved out of plotspec and into its own function, but it's still worth thinking about...