simple TextAnnotation

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            var pm = new PlotModel("Trigonometric functions", "Example using the FunctionSeries")
                         {
                             PlotType = PlotType.Cartesian,
                             Background = OxyColors.White
                         };
            pm.Series.Add(new FunctionSeries(Math.Sin, -10, 10, 0.1, "sin(x)"));
            pm.Series.Add(new FunctionSeries(Math.Cos, -10, 10, 0.1, "cos(x)"));
            pm.Series.Add(new FunctionSeries(t => 5 * Math.Cos(t), t => 5 * Math.Sin(t), 0, 2 * Math.PI, 0.1, "cos(t),sin(t)"));

            TextAnnotation ta = new TextAnnotation();
            DataPoint dp = new DataPoint();
            dp.X = 1;
            dp.Y = 3.5;
            ta.addPoint(dp);

            dp = new DataPoint();
            dp.X = 21;
            dp.Y = 4.5;
            ta.addPoint(dp);
            pm.Annotations.Add(ta);

            plot1.Model = pm;
        }
   }




    public class TextAnnotation : Annotation
    {
        private IList<IDataPoint> dataPoints;

        public void addPoint(IDataPoint dp)
        {
            dataPoints.Add(dp);
        }

        /// <summary>
        ///   Vertical alignment of text (above or below the line).
        /// </summary>
        public VerticalTextAlign TextVerticalAlignment { get; set; }

        /// <summary>
        ///   Horizontal alignment of text.
        /// </summary>
        public HorizontalTextAlign TextHorizontalAlignment { get; set; }


        public override void Render(IRenderContext rc, PlotModel model)
        {
            double angle = 0;
            ScreenPoint position = new ScreenPoint();

            foreach (IDataPoint dp in dataPoints)
            {

                if (dp.X > this.XAxis.ActualMaximum || dp.X < this.XAxis.ActualMinimum)
                    continue;
                if (dp.Y > this.YAxis.ActualMaximum || dp.Y < this.YAxis.ActualMinimum)
                    continue;

                position.X = this.XAxis.Transform(dp.X);
                position.Y = this.YAxis.Transform(dp.Y);

                
                string text = dp.Y.ToString("F1");
                rc.DrawText(
                    position,
                    text,
                    model.TextColor,
                    model.ActualAnnotationFont,
                    model.AnnotationFontSize,
                    FontWeights.Normal,
                    angle,
                    this.TextHorizontalAlignment,
                    this.TextVerticalAlignment);
            }
        }

        public TextAnnotation()
        {
            this.TextHorizontalAlignment = HorizontalTextAlign.Center;
            this.TextVerticalAlignment = VerticalTextAlign.Top;
            dataPoints = new List<IDataPoint>(50);
        }
    }