Garbled marks when moving items in QGraphicsScene

[extralucas]

I have a class which inherits from QGraphicsScene. I use it to display diagrams (inheriting from QGraphicsPolygonItem) and arrows joining the diagrams (inheriting from QGraphicsLineItem). When I move the diagrams, the arrows leave some garbled marks. What should I do? Is there some better, proper word to name the phenomenon?

I attach a photo portraying nature of the problem.