Code

// in MyBehavior.cs
protected override void OnMouseUp(Point mousePosition, MouseButton mouseButton)
{
	DiagramLink link = Diagram.Interaction != null && Diagram.Interaction.Action == Action.Modify ?
		Diagram.Interaction.CurrentItem as DiagramLink : null;

	if (Diagram.Interaction != null && dragSegment == DragLinkSegment.Horizontal)
		mousePosition.X = startPoint.X;

	if (Diagram.Interaction != null && dragSegment == DragLinkSegment.Vertical)
		mousePosition.Y = startPoint.Y;

	base.OnMouseUp(mousePosition, mouseButton);

	if (link != null)
		CleanupLink(link, 2);
}

private void CleanupLink(DiagramLink link, double threshold)
{
	var points = link.ControlPoints;
	for (int i = 0; i < points.Count - 2; ++i)
	{
		if (PointsColinear(points, i, threshold))
		{
			points.RemoveAt(i + 1);
			i--;
		}
	}
	link.UpdateFromPoints(false, true);
}

private bool PointsColinear(PointCollection points, int index, double threshold)
{
	Point p0 = points[index + 0];
	Point p1 = points[index + 1];
	Point p2 = points[index + 2];

	if (Math.Abs(p0.X - p1.X) <= threshold &&
Math.Abs(p1.X - p2.X) <= threshold)
		return true;

	if (Math.Abs(p0.Y - p1.Y) <= threshold &&
Math.Abs(p1.Y - p2.Y) <= threshold)
		return true;

	return false;
} 

Code

		    m_DragSegment = points[segment].Y == points[segment + 1].Y ?
			  DragLinkSegment.Horizontal : DragLinkSegment.Vertical;