Hi Manal,
You can use data labels for this purpose:
   NChart chart = nChartControl1.Charts[0];

    NLineSeries line = new NLineSeries();
    line.Values.Add(10);
    line.Values.Add(20);
    line.Values.Add(30);
    chart.Series.Add(line);

    line.DataLabelStyle.Visible = true;
    line.DataLabelStyle.Format = "<label>";

    // annotate the first data point
    line.Labels.Add("Line Series");

or place an annotation as shown in the All Examples \ Panels \ Annotations example group.

Best Regards,
Nevron Support Team