Combining materialized values in Java GraphDSL.Builder

I know… I wanted to do a marble image generator, like in the Rx sites (just better, maybe with animations). I still think it is possible to write some kind of custom materializer which can provide the graph and logs all the inner events, but my timetable is really full and it would be a lot of work with my current akka streams internal knowledge (I mean the really internal like how the materializer actually work)…