removing Figure background (or, making it transparent)

Hi all,

I'd like use matplotlib to draw shapes on a transparent background using the Agg backend to save .png files. Is this possible?

I've tried

fig._figurePatch.set_alpha(0)

This seems to get halfway, but there's still (opaque) white drawn in the background.

Is there a "recommended way", and, if so, what is it? Otherwise, can we improve matplotlib to allow drawing on transparent backgrounds?

Cheers!
Andrew