How to change the point for gripper?

I am doing custom attribute for component.
I override the RenderGrips method
everything is ok,but when i connet wire to the new grip,the incoming wire cannot attach in the center of the small white circle of gripper…it just connect to the previous position,it seem not ok.
can some one can tell me how to fix this problem?
thank you very much!

You Must ReCompute the bounds of Param in Layout() Method first