Created attachment 696 [details]
screenshot

See screenshot ...

note the first point of Series1, the funcSeries, which has a value of 6 which equals (0+1+2+3) - BUT- this value 6 is not plotted on the last point in this group with value 3. It is plotted on the first point of the next group with value 4. Again, the second point of Series has a value of 22 which equals (4+5+6+7) - again, the last point in this series is 7, but funcSeries plots the value 22 on point 8.

code:
    Line series;
    FastLine funcSeries;
    Add chartTotalFunction;
    private void InitializeChart()
    {
      funcSeries = new FastLine(tChart1.Chart);
      funcSeries.Marks.Visible = true;
      chartTotalFunction = new Add();
      chartTotalFunction.Period = new TimeSpan(0, 12, 0, 0).TotalDays;
      chartTotalFunction.PeriodAlign = Steema.TeeChart.Functions.PeriodAligns.Last;
      chartTotalFunction.PeriodStyle = Steema.TeeChart.Functions.PeriodStyles.Range;
      funcSeries.Function = chartTotalFunction;


      series = new Line(tChart1.Chart);
      DateTime today = DateTime.Today;

      for (int i = 0; i < 20; i++)
      {
        series.Add(today, i);
        today = today.AddHours(3);
      }

      funcSeries.Marks.Visible = true;
      series.Marks.Visible = true;
      funcSeries.DataSource = series;

      tChart1.Axes.Bottom.Labels.Angle = 90;
      tChart1.Axes.Bottom.Increment = new TimeSpan(0, 3, 0, 0).TotalDays;
      tChart1.Axes.Bottom.Labels.DateTimeFormat = "dd/MM/yy hh:mm";
    }