Thanks so much! This is exactly what I needed.

I think in this scenario, I wouldn't recommend using ChartText. Labels don't really have backgrounds, but with a little magic in FillSceneGraph event you can work around the issue:

private void Form1_Load(object sender, EventArgs ev)
{
    ultraChart1.ChartType = ChartType.LineChart;
    ultraChart1.Data.DataSource = new int[,] { { 1, 2, 3, 4, 2, 3, 1, 5 } };
}

private void ultraChart1_FillSceneGraph(object sender, FillSceneGraphEventArgs e)
{
    Polyline line = null;

    foreach (Primitive p in e.SceneGraph)
    {
        if (p is Polyline)
        {
            line = p as Polyline;
            break;
        }
    }

    if (line == null) return;

    foreach (DataPoint point in line.points)
    {
        Text label = new Text();
        string str = point.Value.ToString();
       
        label.SetTextString(str);
        SizeF labelSize = Platform.GetLabelSizePixels(str, label.labelStyle);

        Rectangle bounds = new Rectangle(
            point.point.X - (int)labelSize.Width / 2,
            point.point.Y - (int)labelSize.Height,
            (int)labelSize.Width,
            (int)labelSize.Height);

        label.bounds = bounds;

        Box background = new Box(bounds);
        background.PE.Fill = Color.Yellow;
        background.PE.StrokeWidth = 0;

        e.SceneGraph.Add(background);
        e.SceneGraph.Add(label);
    }
}

If you also want some conditional highlighting, don't add the box to the scenegraph for the point you want to skip