Hi Keith,
By default the oridnal scale on the x axis will inflate the content range by 0.5. In order to show the line exactly at the begin / end of the axis you need to turn off that inflate:
NChart chart = nChartControl1.Charts[0];
NLineSeries line = new NLineSeries();
line.Values.Add(10);
line.Values.Add(20);
line.Values.Add(15);
chart.Series.Add(line);
NOrdinalScaleConfigurator scaleX = chart.Axis(StandardAxis.PrimaryX).ScaleConfigurator as NOrdinalScaleConfigurator;
scaleX.DisplayDataPointsBetweenTicks = false;
scaleX.InflateContentRange = false;
Hope this helps - let us know if you have any questions...