Code

public void SetZoom(double percentage)
{
	var diagramTransform = scrollViewer.TransformToVisual(diagram.DocumentPlane);

	// this point should remain centered when zooming in?
	Point scrollViewerCenter = new Point(
		scrollViewer.ActualWidth / 2, scrollViewer.ActualHeight / 2);
	Point diagramPointAtViewCenter = diagramTransform.Transform(scrollViewerCenter);

	diagram.ZoomFactor = Math.Max(10, Math.Min(percentage, 200));

	Rect clientAreaSize = new Rect(0, 0, scrollViewer.ActualWidth, scrollViewer.ActualHeight);
	Rect clientAreaDiagSize = diagram.ClientToDoc(clientAreaSize);

	// ensure the diagram is always larger than the scrollviewer
	Rect newBounds = GetContentBounds();
	double diffX = Math.Max(0, clientAreaDiagSize.Width - newBounds.Width);
	double diffY = Math.Max(0, clientAreaDiagSize.Height - newBounds.Height);
	newBounds.Inflate(diffX / 2, diffY / 2);
	diagram.Bounds = newBounds;

	// make sure the diagram center is at the scrollviewer center
	diagram.ScrollX = diagramPointAtViewCenter.X - clientAreaDiagSize.Width / 2;
	diagram.ScrollY = diagramPointAtViewCenter.Y - clientAreaDiagSize.Height / 2;
}
 

Code

var diagramTransform = scrollViewer.TransformToVisual(diagram.DocumentPlane);