C# - How to drawLine on top of the chart or another control

Question

I want to draw a line between 2 points on a chart but I cannot use ChartStyle.Line so I tried to use Graphics.DrawLine.

My problem is that I cant draw on top of the chart. How can I solve this?

        PointF pontoantigo = new PointF();

            if (chart1.Series[0].Points.Count > 0)
            {
                 pontoantigo = new PointF((int)chart1.Series[0].Points[0].XValue, (int)chart1.Series[0].Points[0].YValues[0]);
            }


            chart1.Series[0].Points.Clear();

            chart1.Series[0].Points.AddXY(posicao_atual_master.X, posicao_atual_master.Y);
            PointF pontoatual = new PointF((int)chart1.Series[0].Points[0].XValue, (int)chart1.Series[0].Points[0].YValues[0]);

            Pen p = new Pen(Color.Red);
            Graphics g = chart1.CreateGraphics();
            g.DrawLine(p, pontoantigo, pontoatual);

EDIT:

FUNCTION THAT updates the value of the old and new points:

  pontoantigo = new PointF();
            if (chart1.Series[0].Points.Count > 0)
            {
                pontoantigo = new PointF((int)chart1.Series[0].Points[0].XValue, (int)chart1.Series[0].Points[0].YValues[0]);
            }


            chart1.Series[0].Points.Clear();

            chart1.Series[0].Points.AddXY(posicao_atual_master.X, posicao_atual_master.Y);
            pontoatual = new PointF((int)chart1.Series[0].Points[0].XValue, (int)chart1.Series[0].Points[0].YValues[0]);

POSTPAINT:

   private void chart1_PostPaint(object sender, ChartPaintEventArgs e)
    {

        Pen p = new Pen(Color.Red);
        Graphics g = e.ChartGraphics.Graphics;
        g.DrawLine(p, pontoantigo, pontoatual);

    }

STILL NOT WORKING

Answer 1

You can use this function to convert DataPoints to drawing Points:

Point PointFromDataPoint(Chart chart, ChartArea ca, DataPoint pt)
{
    Axis ax = chart2.ChartAreas[0].AxisX;
    Axis ay = chart2.ChartAreas[0].AxisY;
    int x = (int)ax.ValueToPixelPosition(pt.XValue);                    
    int y = (int)ay.ValueToPixelPosition(pt.YValues[0]);
    return new Point(x, y);
}

If you have set the two DataPoints (!!) pontoantigo and pontoatual you can write the PrePaint event:

private void chart1_PrePaint(object sender, ChartPaintEventArgs e)
{
    using (Pen pen = new Pen(Color.Green, 2f))
        e.ChartGraphics.Graphics.DrawLine(pen,
            PointFromDataPoint(chart1, chart1.ChartAreas[0], pontoantigo),
            PointFromDataPoint(chart1, chart1.ChartAreas[0], pontoatual));
}

Here is the result of combining this little post and setting the two DataPoints like this:

 DataPoint pontoantigo = chart1.Series[0].Points.First();
 DataPoint pontoatual  = chart1.Series[0].Points.Last();