Avoid Graphics being clear when forms overlap (cascaded forms)

private void btnBrowse_Click(object sender, EventArgs e)
        {
            //Open folderDialogue at root level
            fbdBrowse = new FolderBrowserDialog();

            //Display the dialogue box
            fbdBrowse.ShowDialog();

            //Get the selected path and display it
            txtDirectory.Text = fbdBrowse.SelectedPath;

            if(Directory.Exists(txtDirectory.Text))
            {
                //Enable the Generate Constellation button
                btnGenConst.Enabled = true;
            }


        }

        private void btnGenConst_Click(object sender, EventArgs e)
        {
            //Get all directories @ the selected path
            string[] sGetDirectory  = Directory.GetFiles(@txtDirectory.Text, "*.cst", SearchOption.AllDirectories);

            //Loop through the directory and create DisplayMdi for file in the directory
            foreach (string sGetpaths in sGetDirectory)
            {
                if (File.Exists(sGetpaths))
                {
                    //Get the name the file
                    string sfilename = sGetpaths.Substring(sGetpaths.LastIndexOf(@"\") + 1);
                    //Call the DisplayMdi.
                    DisplayMdi Display = new DisplayMdi(txtDirectory.Text, sfilename);
                    Display.MdiParent = this;
                    //Display.Text = sfilename.Substring(0,sfilename.LastIndexOf("."));
                    Display.Show();
                    this.LayoutMdi(MdiLayout.Cascade);
                }
            }
            //Disable Generate Constellation until the next browse
            btnGenConst.Enabled = false;
        }

ChildMdi Code:
public partial class DisplayMdi : Form
    {
        Graphics myDrawFillEllipse;
        Graphics myDrawLine;
        Pen myPen;
        SolidBrush myBrush;
        StreamReader strReader;
        Point pntPrevCoodinates;
        Point pntCoodinates;
        private string sDriectory;
        private string sFileName;
        Timer Time = new Timer();
        int iKeepTime = 0;
        int iKeepCount = 0;


        public DisplayMdi(string myDirectory, string myFilename)
        {
            InitializeComponent();
            Time.Interval = 100;
            Time.Tick += new EventHandler(Timer_Tick);
            Time.Start();

            Directory = myDirectory;
            FileName = myFilename;
            Text = myFilename.Substring(0, myFilename.LastIndexOf("."));
            FileStream fsPath = new FileStream(Directory + "\\" + FileName, FileMode.Open, FileAccess.Read);
            strReader = new StreamReader(fsPath);
        }

        public string Directory
        {
            get { return sDriectory; }
            set { sDriectory = value; }
        }
        public string FileName
        {
            get { return sFileName; }
            set { sFileName = value; }
        }
        void Timer_Tick(object sender, EventArgs e)
        {
         
                string input = strReader.ReadLine();
                string[] Coords;
                if (input == null)
                {
                    Time.Stop();
                }
                else
                {
                    myPen = new Pen(Color.Yellow, 2);
                    myBrush = new SolidBrush(Color.Yellow);
                    myDrawFillEllipse = this.CreateGraphics();
                    myDrawLine = this.CreateGraphics();
                    Coords = input.Split(';');

                    pntCoodinates = new Point(Convert.ToInt32(Coords[0]), Convert.ToInt32(Coords[1]));

                    if (iKeepCount == 0)
                    {
                        //Plot the coodinates of the first mouse click
                        myDrawFillEllipse.DrawEllipse(myPen, pntCoodinates.X, pntCoodinates.Y, 4, 4);
                        myDrawFillEllipse.FillEllipse(myBrush, pntCoodinates.X, pntCoodinates.Y, 3, 3);

                    }
                    else
                    {
                        //Plot the coodinates from the first mouse click and draw a line
                        myDrawLine.DrawLine(myPen, pntPrevCoodinates, pntCoodinates);
                        myDrawFillEllipse.DrawEllipse(myPen, pntCoodinates.X, pntCoodinates.Y, 4, 4);
                        myDrawFillEllipse.FillEllipse(myBrush, pntCoodinates.X, pntCoodinates.Y, 3, 3);
                    }

                    pntPrevCoodinates = pntCoodinates;
                    iKeepCount++;
                }
        }

        private void DisplayMdi_Load(object sender, EventArgs e)
        {

        }