I'm currently "not looking at that" for factorio but this tempts me. I will get into it sooner or later im sure.
I've been in need of a diagram app, where you can calculate resources, but they're all outdated now, if anyone has one to recommend.
Rate Calculator lets you select an already-built area and shows you its input and outputs, Factory Planner is pretty neat too with its matrix solver.
Both of those in combination with the Blueprint Lab have been great. Build out the base of your factory in it, save a blueprint, then actually build in your map.
These aren't really diagramming, but I hope they can help.
Another one I like to use for quick diagramming is Factorio Lab , which is a web app.