public Color PolygonLineColor
        {
            set
            {
                Debug.Assert(!InvokeRequired); // Do not call from other threads or use Invoke()
                mi_PolyLinePen = null;
                Invalidate(); // repaint only
            }
            get
            {
                if (mi_PolyLinePen != null) return mi_PolyLinePen.Color;
                else                        return Color.Empty;
            }
        }