This is an issue with the changing of the Legend text by the cursor tool. The correct way of doing this is as follows:

      tChart1.Resize += tChart1_Resize;

    private double xval;

    private void ChangeLegendText(double xValue)
    {
      if (!_isLoaded)
        return;
      xval = xValue;
      for (int i = 0; i < tChart1.Series.Count; i++)
      {
        if (tChart1.Series[i].Active)
        {
          if (tChart1.Series[i] is Steema.TeeChart.Styles.Custom)
          {
            string legendText = tChart1.Series[i].Title + ": " + InterpolateLineSeries(tChart1.Series[i] as Steema.TeeChart.Styles.Custom, xval).ToString("0.00");
            tChart1.Legend.Items[i].Text = legendText;
          }
        }
      }
    }

    void tChart1_Resize(object sender, EventArgs e)
    {
      ChangeLegendText(xval);
    }

    private void cursorTool1_Change(object sender, Steema.TeeChart.Tools.CursorChangeEventArgs e)
    {
      ChangeLegendText(e.XValue);
    }