I would be more inclined to let GraphViz into the process than Dia; the former fits *much* better into a Make-based process.
It is worth observing that there are schema diagramming systems (SchemaSpy is mighty likable) that build diagrams using GraphViz. We have integrated this into internal deployments at Afilias. Rod Taylor's autodoc tool has some minimal Dia support, but it lacks the ability to do layout; if you use Dia output, you'll have to manually drag things around to have the tables *not* all atop one another. I'm a little surprised Dia hasn't integrated in GraphViz to help with that; note that the MacOS diagram tool OmniGraffle (proprietary, considered quite good, competitive with Visio) uses GraphViz to help do automatic object layout. That this is integrated inside an interactive GUI app is pretty wild/cool. I'd think it a fine idea to add rules for .dot files (the usual GraphViz language/format to describe graphs) to the PG Make rules.
