I've just released version 0.6.1 of the Diagrams extension. It makes it possible to use formats other than PNG when using the external web service to render graphs.
I'm not really sure there's a point to having the external service — it does make things harder for sysadmins, and now there's local rendering that works with the AWS extension, perhaps it should be retired. Then again, there's something nice about not filling a wiki's file storage with re-renderable image files. Anyway, I don't have that much time to work on this so it'll probably stay.
The main thing that the local rendering is now missing is support for PlantUML.