you could change it to a Polyline, which does have a splineTension field.  however, with only 2 points, spline tension will have no effect.

possibly the only solution for this is to use a Path primitive and draw the curve as a bezier spline inside the GraphicsPath object.  calculating the control points for the bezier spline is a very tricky problem, though.

you could also try adding a GraphicsContext to the scene before the Polyline, using the GraphicsContext to set the clip bounds beforehand, so that only the space between the two points is visible.

here's some code to give you an idea of what to do

 private void ultraChart1_FillSceneGraph(object sender, FillSceneGraphEventArgs e)
        {
            PrimitiveCollection primitivesToAdd = new PrimitiveCollection();
            foreach (Primitive p in e.SceneGraph)
            {
                PointSet pS = p as PointSet;
                if (pS != null)
                {
                    for (int i = 0; i < pS.points.Length - 1; i++)
                    {
                        Line lineToAdd = new Line(pS.points[i].point, pS.points[i + 1].point);
                        if (i % 2 == 0)
                        {
                            lineToAdd.PE.Stroke = Color.Pink;
                        }
                        else
                        {
                            lineToAdd.PE.Stroke = Color.Lime;
                        }
                        primitivesToAdd.Add(lineToAdd);
                    }
                }
            }
            e.SceneGraph.AddRange(primitivesToAdd.ToArray());
        }

I see the SceneGraph object, but dont see any member that gets me to a set of PointSet objects. Am i following correctly? 

Do I add Lines or modify the existing Lines?

thx