Code

void DistributeLinks(DiagramNodeCollection nodes)
{
	Dictionary<DiagramLink, int> linkLevels = new Dictionary<DiagramLink, int>();
	int levels = GetLinkLevels(nodes, linkLevels);

	float top = nodes[0].Bounds.Top;
	float levelDist = nodes[0].Bounds.Height / (levels + 1);
	foreach (DiagramLink link in linkLevels.Keys)
	{
		int level = linkLevels[link];
		float y = top + (level + 1) * levelDist;
		int i1 = 0, i2 = 0;
		if (nodes.Contains(link.Origin) && nodes.Contains(link.Destination))
		{
			i1 = 0;
			i2 = link.ControlPoints.Count;
		}
		else if (nodes.Contains(link.Origin))
		{
			i1 = 0;
			i2 = 2;
		}
		else if (nodes.Contains(link.Destination))
		{
			i1 = link.ControlPoints.Count - 2;
			i2 = link.ControlPoints.Count;
		}
		for (int i = i1; i < i2; ++i)
		{
			PointF point = link.ControlPoints[i];
			point.Y = y;
			link.ControlPoints[i] = point;
		}
		link.UpdateFromPoints();
	}
	diagram.Invalidate();
}

int GetLinkLevels(DiagramNodeCollection nodes, Dictionary<DiagramLink, int> linkLevels)
{
	List<DiagramLink> unassignedLinks = new List<DiagramLink>();
	foreach (DiagramLink link in diagram.Links)
	{
		if (nodes.Contains(link.Origin) || nodes.Contains(link.Destination))
			unassignedLinks.Add(link);
	}

	int currentLevel = 0;

	while (unassignedLinks.Count > 0)
	{
		IntervalGraph graph = GraphFromLinks(unassignedLinks, nodes);
		graph.SetItemIndices();

		List<Vertex> independentSet = graph.MaxWeightedIndependentSet();
		List<DiagramLink> nonOverlapping = new List<DiagramLink>();
		foreach (Vertex vertex in independentSet)
		{
			DiagramLink link = unassignedLinks[vertex.index];
			linkLevels[link] = currentLevel;
			nonOverlapping.Add(link);
		}
		foreach (DiagramLink link in nonOverlapping)
			unassignedLinks.Remove(link);
		currentLevel++;
	}

	return currentLevel;
}

IntervalGraph GraphFromLinks(List<DiagramLink> links, DiagramNodeCollection nodes)
{
	List<Interval> linkIntervals = new List<Interval>();
	foreach (DiagramLink link in links)
	{
		int last = link.ControlPoints.Count - 1;
		if (nodes.Contains(link.Origin) && nodes.Contains(link.Destination))
		{
			PointF p1 = link.ControlPoints[0];
			PointF p2 = link.ControlPoints[last];
			linkIntervals.Add(
				new Interval(Math.Min(p1.X, p2.X), Math.Max(p1.X, p2.X)));
		}
		else if (nodes.Contains(link.Origin))
		{
			PointF p1 = link.ControlPoints[0];
			PointF p2 = link.ControlPoints[1];
			linkIntervals.Add(
				new Interval(Math.Min(p1.X, p2.X), Math.Max(p1.X, p2.X)));
		}
		else if (nodes.Contains(link.Destination))
		{
			PointF p1 = link.ControlPoints[last];
			PointF p2 = link.ControlPoints[last - 1];
			linkIntervals.Add(
				new Interval(Math.Min(p1.X, p2.X), Math.Max(p1.X, p2.X)));
		}
	}
	return new IntervalGraph(linkIntervals);
}