Look up sound cue and sound class editor source code, is one of simplest graph editors.
https://github.com/EpicGames/UnrealEngine/tree/release/Engine/Source/Editor/SoundClassEditor
https://github.com/EpicGames/UnrealEngine/tree/release/Engine/Source/Editor/SoundCueEditor
Importent note is that graph is like text code, just a discription… blueprint 
using that graph you compose a data which will be used to compose data used in gameplay, for example sound cue use only one graphnode class to hold sound cue node data and when you save it spawn sound cue nodes objects and change them together
You probably also interested in creating assets type, you start with making AssetToolAction class which descibe the asset and actions you can do. Engine holds all assetstoolaction classes in one module but you can place it on any module:
As those classes are not UObject you need to manualy register them on module load for make edito to support asset:
But thats only to support asset in content browser, make import or create new asset you need to make UFactory class… but i gonna leave that as homework for you… it not that hard to figure if you look on other UFactory classes