_figure_cache problem if figure is closed

[jfburkhart]

Usage of _get_figure_cache and _figure_cache is a nice solution for keeping figures active for plotting. However, if working interactively in Ipython, and the figure is closed, the next time one tries to plot using the same parameters, there is an error something like:
KeyError: <matplotlib.axes._axes.Axes object at 0x7f4edb4b17f0>
resulting from the figure no longer being available.
The "working" solution is to pop the figure from the figure cache, for example:

rf.plotting._figure_cache.pop('defaultNoneNone')
But this is certainly not the intention.