Backend selection without $DISPLAY available

[akhmerov]

Availability of $DISPLAY doesn't influence the logic of selecting the default backend. This makes usage of pyplot more involved via ssh and in environments without displays. Right now executing

import os
del os.environ['DISPLAY']
from matplotlib import pyplot
pyplot.plot([0, 1])

Will result in an error. Perhaps a fallback should be used if there is no display available?

[mdboom]

The default backend is selected at compile time, not at run time. I think it would actually be more confusing if the backend were selected through some automated mechanism (i.e. when in doubt, don't guess). The backend can be selected manually using the matplotlibrc, and Agg will work without a DISPLAY.