All good points. Clearly the use case I'm reacting to at the moment is the demo/working-with-in-process-model issue, but having a slick way to chart final-data-collection is very handy too.
A checkbox could be the quickest, easiest option to facilitate both usages.
I would envision a dialogue box that lets you manually specify the image dimensions/proportions, so users who are exporting PDFs or manually tiling charts across the screen and want them to be the same size, etc, have that option. That'd be a bit more work though -- a checkbox would have the same effect.
IMO the default behavior ought to be scale-on-drag & fill whole window. You'd also need a mechanism to auto-adjust text size.