private void GraphControl_KeyDown(object sender, KeyEventArgs e)
        {
            switch (e.Key)
            {
            case Key.LeftShift:
                currentMode = ManipulateMode.Vertical;
                Cursor      = Cursors.SizeNS;
                break;

            case Key.LeftCtrl:
                currentMode = ManipulateMode.Horizontal;
                Cursor      = Cursors.SizeWE;
                break;

            case Key.Delete:
            {
                if (selectedLinePoint != null)
                {
                    RemovePoint(selectedLinePoint);
                    selectedLinePoint = null;
                }
            }
            break;
            }
        }
 private void GraphControl_KeyUp(object sender, KeyEventArgs e)
 {
     switch (e.Key)
     {
     case Key.LeftShift:
     case Key.LeftCtrl:
         currentMode = ManipulateMode.All;
         break;
     }
 }