Saving a figure to StringIO or similar

I am writing a web server app that creates charts among

    > other things. I am trying to get rid of the temporary file
    > that I use to transmit the figures created with matplotlib
    > to the actual web server. Although print_figure says "If
    > filename is a fileobject, write png to file object (thus
    > you can, for example, write the png to stdout)" I can't
    > successfully write anything to stdout. Anyone knows an
    > example or can give me some hint what I can do to get rid
    > of the tempfile?

Short answer: no known way to do this currently, though we'd like to
figure it out. As far as I know (and could very well be wrong)
libpng requires a FILE*, which StringIO and cStringIO do not provide.