Issues with matplotlib notebook backend

[ixjlyons]

• I noticed in submitting an assignment through nbgrader that plots don't show up (I think they work with %matplotlib inline but need to test)
• The line >>> trajectories['book_angle'].plot(); in the book balancing notebook updates the plot above instead of creating a new figure

I've noticed a few other somewhat buggy behaviors compared to inline, but can't remember them specifically (iirc similar to the second point above). The animation doesn't appear to work with the inline backend at the moment though.

[moorepants]

Yes. I noticed the same thing. It is odd. Also the plots don't seem to be showing up after running the execute_notebook.sh script.

[moorepants]

I think we've got this mostly working.