connecting circles dynamically

[waterfall]

Hi all,
Im having some problem in my application .i want to take input of tasks and preceding activities. What I want to do is to draw circles for the given number of tasks and connect them according to their preceding activities.e.g, if user input 1,2 for preceding activity 3 then it connects tasks 1 and task2. I have drawn the circles but have problem in connecting them. here’s my code..

protected void Button1_Click(object sender, EventArgs e)
    {
        //values of textboxes stored in array
        ArrayList tasks = new ArrayList();
        tasks.Add(Task1TB.Text);
        tasks.Add(Task2TB.Text);
        tasks.Add(Task3TB.Text);
        //Preceding activities stored in array
        ArrayList pa = new ArrayList();
        pa.Add(PA1TB.Text);
        pa.Add(PA2TB.Text);
        pa.Add(PA3TB.Text);

        Bitmap Bmp1;
        Graphics Gfx;
        Bmp1 = new Bitmap(600, 500);
        Font Fnt;
        int intFontSize = 16;
        Fnt = new System.Drawing.Font("Arial", intFontSize, FontStyle.Bold);

        int xpos = 50;
        int ypos = 50;
        int xpos2 = 50 + 10 * 2;
        int ypos2 = 50 + 10 % 2;

        //draw circles for each task
        for (int i = 1; i <= tasks.Count; i++)
        {
            Gfx = Graphics.FromImage(Bmp1);
            Gfx.DrawString("Task" + i, Fnt, Brushes.White, xpos - 10 * 2, ypos - 10 * 2);
            Gfx.DrawEllipse(Pens.DarkOrange, xpos, ypos, 75, 75);

            xpos = xpos + 80;
            ypos = ypos + 90 * 2;

            //checks preceding activity and draws line accordingly
            //e.g. if task2 depands on task1 a line coonects from tasks2 to task1.
            //if task3 depends on task2 and task1 then task3 connects to task2 and task1
                if (pa[2] == "1")
                {

                    Gfx.DrawLine(Pens.White, xpos, ypos, xpos2, ypos2);
                }

        }
        Bmp1.Save(Response.OutputStream, ImageFormat.Gif);

    }