If you need auto-layout, it's hard to do much better than graphviz. The creators of graphviz have some good papers on what it takes to do that right. They'll discourage you from wanting to re-invent that particular wheel, if you read them. Tricky bit here is, depending on your exact graph and constraints, it's not exactly fully solvable in the general case, if you want to have no overlap or no crossing lines under any circumstances whatsoever. Still, you'll struggle to do better, and there aren't actually a ton of implementations out there other than Graphviz that are anywhere near as good (at least, as of a couple years ago).
Templating or building programmatically? Great, it's just text. Generating on demand? Decent, it's fairly fast.
If you need pretty, and I mean really pretty, and especially pretty and interactive... well it's less enjoyable for that, but then nothing that is enjoyable for that purpose is much good at the other points above.