Hi Irina,
You can achieve this with the help of a second point series that has data points offset by 0,5 from the line series points. For example:

   NChart chart = nChartControl1.Charts[0];

   NStepLineSeries line = new NStepLineSeries();
   line.DataLabelStyle.Visible = false;

   line.Values.Add(10);
   line.Values.Add(20);
   line.Values.Add(30);

   chart.Series.Add(line);

   NPointSeries point = new NPointSeries();
   point.Size = new Nevron.GraphicsCore.NLength(0);
   point.UseXValues = true;

   for (int i = 0; i < line.Values.Count - 1; i++)
   {
    point.Values.Add(line.Values[i]);
    point.XValues.Add(i + 0.5);
   }

   chart.Series.Add(point);

Hope this helps - let us know if you meet any problems.

Best Regards,
Nevron Support Team