using the bottommost axes as the last axes instance
This allows issue #60 to to be resolved. I am unaware of the consequences of this, though.
This is the image generated with the same code in #60 using the patch I submitted in this PR, for your reference (with some added styling of the right hand figure).

Two of the tests in the python notebook fail:
- The first one that shows the grid. The bottom grid is drawn correctly, but the grids above that are drawn on top of the data. I am working on that
- The Unknown Pleasures album cover is not being displayed. I am not sure why. I will look into that after I figure out the grid.
If you happen to want to accept this as a PR and have any insight, please let me know.
Hey! Thanks for the contribution. I'll take a look when I have the chance.
There are still some outlying problems:
- The Unknown Pleasures example, for some reason, is not rendering in the notebook for me.
- Grid lines in density estimate plots overlay the density estimate. That is not true for histograms.
I am looking into both.
Well, I figured out the Unknown Pleasures issue, all that's left is the grid zorder issue.