If you don't find a better solution, I'd be inclined to emit a .dot file and then feed it to one of the Graphviz tools (like dot). The file format is really simple, and the tools can generate a variety of formats including PNG, SVG, PostScript and even image maps (for creating clickable graphs on web pages).
This approach probably only works for you if you want these graphs for your own use (eg: debugging) or you're generating them on a server. Calling out to a separate tool in a client app might have usability and/or licensing issues.
This isn't really a common-lisp specific solution, but I've used this approach from a few languages in the past.