Hi Manal,
You can assign different stroke style per line segment using the BorderStyles collection of the series - for example:
NChart chart = nChartControl1.Charts[0];
NLineSeries line = new NLineSeries();
line.Values.Add(1);
line.BorderStyles[0] = new NStrokeStyle(Color.Red);
line.Values.Add(2);
line.Values.Add(3);
chart.Series.Add(line);
will draw a line where the first segment is in different color.
Best Regards,
Nevron Support Team