Export PNG with fixed width?

[ml knorke in-berlin de (ml)]

Hi,

I like to export a diagram in PNG format with
a specific width, e.g. 400 pixels.  Is this
possible?  I would prefer a command line over
interactive usage, like

$ dia foo.dia --export=foo.png --width=400

or sth. like that.

Thanks in advance!

Cheers!