public Rectangle GetVisibleRectangleInGraph() {
var t = Transform.Inverse;
var p0 = new Point(0, 0);
var vp = GetCanvasRenderViewport();
var p1 = new Point(vp.Width, vp.Height);
var rect=new Rectangle(t*p0, t*p1);
if (GeomGraph == null)
return rect;
return rect.Intersection(GeomGraph.BoundingBox);
}