Code

void SetZoomFactor(double zoomFactor, Point anchor)
{
	var pivot = DocToClient(anchor);
	var scroll = DocToClient(new Point(diagram.ScrollX, diagram.ScrollY));
	var offset = new Point(pivot.X - scroll.X, pivot.Y - scroll.Y);

	diagram.ZoomFactor = zoomFactor;

	offset = ClientToDoc(offset);
	diagram.ScrollX = anchor.X - offset.X;
	diagram.ScrollY = anchor.Y - offset.Y;
}

Point DocToClient(Point point)
{
	return diagram.DocumentPlane.TransformToVisual(diagram).Transform(point);
}

Point ClientToDoc(Point point)
{
	return diagram.TransformToVisual(diagram.DocumentPlane).Transform(point);
}

private void OnMouseWheel(object sender, MouseWheelEventArgs e)
{
	SetZoomFactor(
		diagram.ZoomFactor + e.Delta / 20.0,
		e.GetPosition(diagram.DocumentPlane));
}

void OnKeyDown(object sender, KeyEventArgs e)
{
	var v = diagram.Viewport;
	var center = new Point(v.X + v.Width / 2, v.Y + v.Height / 2);

	switch (e.Key)
	{
		case Key.Add:
			SetZoomFactor(diagram.ZoomFactor * 1.2, center);
			break;

		case Key.Subtract:
			SetZoomFactor(diagram.ZoomFactor / 1.2, center);
			break;
	}
}