code: @SuppressWarnings("unchecked")
  public void mouseDragged(MouseEvent e) {
      if(checkModifiers(e)) {
          if(startVertex != null) {
              transformEdgeShape(down, e.getPoint());
              transformArrowShape(down, e.getPoint());
          }
          VisualizationViewer<String,String> vv =
              (VisualizationViewer<String,String>)e.getSource();
          vv.repaint();
      }
  }
docs: If startVertex is non-null, stretch an edge shape between
  startVertex and the mouse pointer to simulate edge creation