Created attachment 94 [details]
button10 click result

Pressing the following button:
        private void button10_Click(object sender, EventArgs e)
        {

          Steema.TeeChart.Styles.Line line = new Steema.TeeChart.Styles.Line(tChart1.Chart);
          line.LinePen.Width = 2;

          tChart1.Series.Add(line);

          tChart1.Series[0].XValues.DateTime = true;

          tChart1.Series[0].Add(new DateTime(2009, 7, 1, 0, 0, 0), 6525.4632);
          tChart1.Series[0].Add(new DateTime(2009, 7, 1, 0, 30, 0), 6325.000016);
        }

gives the attached image button10.png. Pressing the two buttons sequentially:

        private void button10_Click(object sender, EventArgs e)
        {

          Steema.TeeChart.Styles.Line line = new Steema.TeeChart.Styles.Line(tChart1.Chart);
          line.LinePen.Width = 2;

          tChart1.Series.Add(line);

          tChart1.Series[0].XValues.DateTime = true;
        }

        private void button11_Click(object sender, EventArgs e)
        {
          tChart1.Series[0].Add(new DateTime(2009, 7, 1, 0, 0, 0), 6525.4632);
          tChart1.Series[0].Add(new DateTime(2009, 7, 1, 0, 30, 0), 6325.000016);
        }

gives the attached image button11.png.

The difference between the two images is that the space between the left axis and the edge of the Chart component is bigger in the first case than the second.