setting initial background size

[ronman]

I'm trying to get Canviz to define a large background canvas so that nodes can be pinned anywhere on the screen (layout by Graphviz' fdp).  Right now, the background is initially the size of the first node plus its padding.  The first node can't be placed mid-screen, but defaults to upper left.  The second node can't be placed anywhere on the screen but must be close to the first, and the background canvas is just large enough to encompass just those two.

Can you give me a hint where in the canviz code I can start looking to modify for this behavior?  I've spent a few hours on it but haven't gotten what I want.   Currently looking primarily at var bbRect.

Thanks,