Comment by kryptn

I've used a combination of three mods to help planning.

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.

https://mods.factorio.com/mod/RateCalculator

https://mods.factorio.com/mod/factoryplanner

https://mods.factorio.com/mod/BlueprintLab_design

These aren't really diagramming, but I hope they can help.