why d curve is again coming back to zero value

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C#

I'm reading from textboxes and writing to a log.txt file first. Reading from that log.txt file respective to time and date format and displaying in richtextbox and drawing a graph plot for those values. Now in the plot the curve is returning back to zero at last. Please help so dat the plot at last must not come back to zero value

private void button1_Click(object sender, EventArgs e)
       {
           OpenFileDialog ofd = new OpenFileDialog();
           if (ofd.ShowDialog() == DialogResult.OK)
           {
               StreamWriter sw = new StreamWriter(File.OpenWrite(ofd.FileName));
               sw.WriteLine(DateTime.Now);
               sw.WriteLine(textBox1.Text);
               sw.WriteLine(textBox2.Text);
               sw.WriteLine(textBox3.Text);
               sw.WriteLine(textBox4.Text);
               sw.WriteLine(textBox5.Text);
               sw.Dispose();
           }

        }
       List<double> lines = new List<double>();
       private void button2_Click(object sender, EventArgs e)
       {

           string input = textBox6.Text;
           DateTime dt = Convert.ToDateTime(input);
           string str;
           StreamReader s = File.OpenText("c:\\log.txt");
           richTextBox1.Clear();
           zedGraphControl1.GraphPane.CurveList.Clear();

           while ((str = s.ReadLine()) != null)
           {
               if (str.Contains(input))
               {
                   str = s.ReadLine();
                   try
                   {
                       while (!str.Contains('/'))
                       {
                           lines.Add(Convert.ToDouble(str));
                           richTextBox1.Text += str + "\n";
                           str = s.ReadLine();
                       }
                   }
                   catch { }
               }
           }
           s.Dispose();
       }
       private void button3_Click(object sender, EventArgs e)
       {
           double[] z = new double[30];
           double[] x = new double[30];

           int h = 0;
           try
           {
               foreach (double line in lines)
               {
                   MessageBox.Show(line.ToString());

                       z[h] = line;
                       x[h] = (double)h;
                       h++;
                }
            }
            catch
           {

           }
           zedGraphControl1.GraphPane.CurveList.Clear();

           // GraphPane object holds one or more Curve objects (or plots)
           GraphPane myPane = zedGraphControl1.GraphPane;

           // PointPairList holds the data for plotting, X and Y arrays
           PointPairList spl1 = new PointPairList(x, z);


           // Add cruves to myPane object
           LineItem myCurve1 = myPane.AddCurve("Sine Wave", spl1, Color.Blue, SymbolType.None);


           myCurve1.Line.Width = 3.0F;

           myPane.Title.Text = "My First Plot";

           // I add all three functions just to be sure it refeshes the plot.
           zedGraphControl1.AxisChange();
           zedGraphControl1.Invalidate();
           zedGraphControl1.Refresh();
           }