I was frustrated that I could align static images, but not dynamically created Graphviz components. So here's a patch to enable that.
Ideally, '.. graphviz::' could be "subclassed" somehow such that it could generate the image then pass off the image to the '.. figure::' functionality, which could then handle :align:, but alas. That was more work. Here's a small change that I think at least handles the HTML case for aligning just-in-time Graphviz created images.