private void UpdateRotation(double deltaR)
{
// calculate new rotated isometric projection
rotation = ((rotation + deltaR) + 360) % 360;
var transform = Projections.Isometric.Clone();
var rotate = Matrix2D.CreateRotateInstance((Math.PI * rotation) / 180);
rotate.Multiply(transform, MatrixOrder.Append);
// set new projection keeping the center of the content rect
graphControl.UpdateContentRect();
var center = graphControl.ContentRect.Center;
graphControl.Projection = rotate;
// update z-order of model items according to new rotation
((IsometricGraphModelManager)graphControl.GraphModelManager).Update();
graphControl.Center = center;
graphControl.Invalidate();
}