matplotlib.axes._process_plot_format fix.

matplotlib.axes._process_plot_format doesn’t work with matplotlib
1.4.1, as _process_plot_format is a method on matplotlib._axes, not
matplotlib.axes. Edited call to axes._process_plot_format to be
axes._axes._process_plot_format. This may not be safe with older
versions of matplotlib, but don’t have a good way to test or evaluate a
better fix.