Code

private void zoomToFitToolStripMenuItem_Click(object sender, EventArgs e)
{
	diagram.ResizeToFitItems(5);
	diagramView.ZoomToFit();

	// center diagram in view
	RectangleF bounds = diagram.Bounds;
	RectangleF viewport = diagramView.ClientToDoc(diagramView.ClientRectangle);
	PointF diagCenter = GetCenter(bounds);
	diagramView.ScrollTo(
		diagCenter.X - viewport.Width / 2,
		diagCenter.Y - viewport.Height / 2);

	// make sure diagram fills view
	float dx = 0, dy = 0;
	if (viewport.Width > bounds.Width)
		dx = (viewport.Width - bounds.Width) / 2;
	if (viewport.Height > bounds.Height)
		dy = (viewport.Height - bounds.Height) / 2;
	bounds.Inflate(dx, dy);
	diagram.Bounds = bounds;
}

PointF GetCenter(RectangleF r)
{
	return new PointF(r.X + r.Width / 2, r.Y + r.Height / 2);
} 

Code

private void OnMouseWheel(object sender, MouseEventArgs e)
{
	PointF docPointAtMousePos = diagramView.ClientToDoc(e.Location);

	int factorToIncrese = e.Delta * SystemInformation.MouseWheelScrollLines / 120;
	diagramView.ZoomFactor += factorToIncrese;

	PointF newPointAtMousePos = diagramView.ClientToDoc(e.Location);
	diagramView.ScrollX -= newPointAtMousePos.X - docPointAtMousePos.X;
	diagramView.ScrollY -= newPointAtMousePos.Y - docPointAtMousePos.Y;
}