如何点击添加或拖动D3?

I get the impression this question is so simple nobody has bothered to make a demo of it, but I don't know enough D3 (yet) to see what I'm doing wrong. The behavior I'm looking for is if the user clicks where there's not a circle it will create one there, and if they drag an existing circle a new one will not be created, but the one they drag will move.I get the impression this question is so simple