private void AddMinMaxLabel(NAxis axis, double min, double max)
   {
    NAxisConstLine constLine1 = new NAxisConstLine
    {
      StrokeStyle = { Color = Color.Gray },
      Value = min,
      TextAlignment = ContentAlignment.MiddleLeft,
      Text = " Min " + min.ToString("0.000")
    };
    constLine1.TextStyle.FontStyle.Style = FontStyle.Bold;
    NAxisConstLine constLine2 = new NAxisConstLine
    {
      StrokeStyle = { Color = Color.Gray },
      Value = max,
      TextAlignment = ContentAlignment.TopLeft,
      Text = " Max " + max.ToString("0.000")
    };
    constLine2.TextStyle.FontStyle.Style = FontStyle.Bold;
    axis.ConstLines.Add(constLine1);
    axis.ConstLines.Add(constLine2);
   }