Hi,

The pgf output does not support a stream as illustrated by this snippet

import sys
plt.plot([1,2])
plt.savefig(sys.stdout, format='pgf')

It returns:

ValueError: saving pgf to a stream is not supported, consider using the pdf option of the pgf-backend

Other formats like png, eps, ps, svg support it. I don't see any good reason to do not support streams for pgf and I guess it is a bug :)

Thanks.