In EditorGUI, how to implement a large Editable Panel like what you can do in an UML diagram?

Hi Unity pros! I am writing my own Unity plugin and building customized window for easy use. My question is how to implement an "UML diagram" like function that you can drag some blocks and lines on it. Which may seems like this Link: Dialogue System for Unity The first picture (right after Youtube link) in the preview part.

I have looked through the whole EditorGUI & EditorGUILayout but cannot come out a way to implement that. Does anyone have any idea of that? No need to be detailed but only a key point is enough. Thanks in advance.