matplotlib.pyplot throws a RuntimeError when no display is present.  One
way around that is to set the backend to AGG.  But we don't want AGG when
we do have a display.  So we only want to set it opportunistically.  But
it also has to be set before anything else is imported.  Kind of convoluted.

So we really just import pyplot as opportunistically as possible, and then
rely on setting AGG backend when we're sure we don't need it in main().