Poorly stretched plots in plotting docs with latest sphinx-rtd-theme

This is now showing up on ReadTheDocs:
http://xarray.pydata.org/en/latest/plotting.html#facetgrid-objects

Example:
<img width="596" alt="screen shot 2018-06-01 at 8 45 06 am" src="https://user-images.githubusercontent.com/1217238/40850056-77734832-6578-11e8-8bdc-75b902dd209d.png">
