Code

LayeredLayout ll = new LayeredLayout();
ll.Arrange(diagram);
diagram.ResizeToFitItems(10);

foreach (DiagramNode node in diagram.Nodes)
{
	DiagramNodeCollection childNodes = new DiagramNodeCollection();
	foreach (DiagramLink link in node.OutgoingLinks)
	{
		DiagramNode dest = link.Destination;
		if (Math.Abs(node.Bounds.Bottom + ll.LayerDistance - (dest.Bounds.Y)) < 0.1)
			childNodes.Add(dest);
	}

	ArrayList sortedByX = new ArrayList(childNodes);
	sortedByX.Sort(new XComparer());
	RectangleF[] sortedRects = new RectangleF[childNodes.Count];
	for (int i = 0; i < childNodes.Count; ++i)
		sortedRects[i] = ((DiagramNode)sortedByX[i]).Bounds;
	for (int i = 0; i < childNodes.Count; ++i)
		childNodes[i].Bounds = sortedRects[i];
}

public class XComparer : IComparer
{
	public int Compare(object x, object y)
	{
		DiagramNode n1 = x as DiagramNode;
		DiagramNode n2 = y as DiagramNode;

		if (n1.Bounds.X < n2.Bounds.X) return -1;
		if (n1.Bounds.X > n2.Bounds.X) return 1;
		return 0;
	}
}