I am very happy to announce that Tech Tree Designer plugin is already in the marketplace: https://www.unrealengine.com/marketplace/slug/18b7eb4966ca47f48f27f243c97bb785
It provides easy way to design your Skill / Tech trees in Unreal Graph Editor. There is nothing like this plugin in UE4 marketplace which offers one place do define both logic and look of your Tree.
Check documentation here:
https://docs.google.com/document/d/1ZEXM7QMiv2RURMgrcfSiPEbthYC0mu79rwewh1HTAUU