 the validation code for customization using
Matplotlib's rc settings.

Each rc setting is assigned a function used to validate any attempted changes
to that setting.  The validation functions are defined in the rcsetup module,
and are used to construct the rcParams global object which stores the settings
and is referenced throughout Matplotlib.

The default values of the rc settings are set in the default matplotlibrc file.
Any additions or deletions to the parameter set listed here should also be
propagated to the :file:`lib/matplotlib/mpl-data/matplotlibrc` in Matplotlib's
root source directory.
