Hi,
It doesn't seem that there is much interest in a Visio backend 
I saw
John saying that the list of contributors was already huge, etc. so I
put up my own "local" version of matplotlib, so I can continue easily
working on the Visio backend.
If you are curious about the progress, you can have a look at
http://www.hamletg.org/wiki/index.php/Matplotlib
BR,
Michel