This is a bug in how we pass aspect ratio in to plots using matplotlib's imshow command. I've opened Trac 15315 for this.
This is a bug in how we pass aspect ratio in to plots using matplotlib's imshow command. command, and thank you for finding it; the typical use cases have been square inputs! I've opened Trac 15315 for this.this. I tried to look for a few workarounds, but the things I tried for accessing the aspect ratio of the underlying mpl figure wouldn't propagate (for the cognoscenti, even doing savefig now doesn't work in Sage).