Графика в VB 2010.drawline

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Я пишу небольшую программу-заставку для рисования линий на экране с помощью таймера.
Для этого из файла считываются две точки данных A &B и вычисляются координаты (x,y) в пикселях для A &B.
Линия от точки А до точки в рисуется с помощью метода graphics. drawline. Пока все работает нормально.
Следующая линия от точки в ДО С рисуется аналогично.
Проблема в том, что при рисовании линии BC линия AB не сохраняется на экране.
Как я могу нарисовать отрезки линий BC, CD, DE и так далее с помощью таймера, сохраняя при этом предыдущие ?
Я использую VB 2010.

По сути, я рисую линию, состоящую из многочисленных сегментов, где каждый сегмент должен быть нарисован с помощью события тика таймера.

Что я уже пробовал:

Я пробовал не использовать "обновить" в timer. tick sub.

вот код

Imports Microsoft.Win32
Imports Microsoft.VisualBasic
Imports System.Math
Imports System.Drawing.Drawing2D

Public Class frmscr
    Dim greenPen2 As New Pen(Color.Green, 5)
    Dim grayPen1 As New Pen(Color.Gray, 1)

    Dim WithEvents tmrClock As New Timer

    Public point1 As New Point
    Public point2 As New Point

    Private Sub frmscr_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        'Read input data file,reads array of GR
        Call ReadTxtInputFile()

        'Get screen size in pixels
        scrW = Screen.PrimaryScreen.Bounds.Width.ToString
        scrH = Screen.PrimaryScreen.Bounds.Height.ToString
        Windows.Forms.Cursor.Hide()

        'initialize variable i
        i = 1

        'Compute screen x,y of start point in pixels
        'where xo1p is x offset in pixels
        'gr(0) is 1st point of array
        't1Wp is width of 1st vertical track
        'grmax & grmin are max & min values of parameter array value gr(i)
        xGR0 = xo1p + t1Wp * (GR(0) - GRmin) / (GRmax - GRmin)
        yGR0 = 0

        'set initial values of second point & its backup value
        xGR1 = xGR0
        xGR2 = xGR0
        yGR1 = yGR0
        yGR2 = yGR0


        ' Set Pen Styles
        greenPen2.DashStyle = Drawing2D.DashStyle.Solid
        grayPen1.DashStyle = Drawing2D.DashStyle.DashDotDot

        'timer
        tmrClock.Enabled = True
        tmrClock.Interval = 50
        tmrClock.Start()


    End Sub

    Private Sub frmscr_FormClosing(sender As Object, e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
        Windows.Forms.Cursor.Show()
    End Sub

    Private Sub frmscr_KeyDown(sender As Object, e As System.Windows.Forms.KeyEventArgs) Handles Me.KeyDown
        Me.Close()
    End Sub

    Private Sub frmscr_KeyPress(sender As Object, e As System.Windows.Forms.KeyPressEventArgs) Handles Me.KeyPress
        Me.Close()
    End Sub

    Private Sub frmscr_MouseClick(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseClick
        Me.Close()
    End Sub

    Private Sub frmscr_MouseDoubleClick(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseDoubleClick
        Me.Close()
    End Sub

    Private Sub frmscr_MouseMove(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseMove
        Static OldX As Integer
        Static OldY As Integer
        'Determines whether the mouse was moved and whetherthe movement was large.
        ''If so, the screen saver is ended.
        If (OldX > 0 And OldY > 0) And (Math.Abs(e.X - OldX) > 3 Or Math.Abs(e.Y - OldY) > 3) Then
            Me.Close()
        End If
        ''Assigns the current X and Y locations to OldX and OldY.
        OldX = e.X
        OldY = e.Y
    End Sub
    Private Sub DrawGrids(ByVal e As PaintEventArgs)

        ' Define Vertical grid points : track width in % : default values
        t1W = 15

        'track width in pixels
        t1Wp = t1W * scrW / 100

        'track x offset in pixels
        xo1p = 0


        ' Create points that define line.
        'vertical grid
        Dim ii As Integer

        For ii = 1 To 4 Step 1
            Dim point1 As New Point(t1Wp / 10 * ii, 0)
            Dim point2 As New Point(t1Wp / 10 * ii, scrH)
            e.Graphics.DrawLine(grayPen1, point1, point2)
        Next ii
        For ii = 6 To 9 Step 1
            Dim point1 As New Point(t1Wp / 10 * ii, 0)
            Dim point2 As New Point(t1Wp / 10 * ii, scrH)
            e.Graphics.DrawLine(grayPen1, point1, point2)
        Next ii

        For ii = 0 To 10 Step 5
            Dim point1 As New Point(t1Wp / 10 * ii, 0)
            Dim point2 As New Point(t1Wp / 10 * ii, scrH)
            e.Graphics.DrawLine(Pens.Blue, point1, point2)
        Next ii


        'horizontal grid
        nhThk = scrH / 75


        For ii = 1 To nhThk Step 1
            Dim point1 As New Point(0, 75 * ii)
            Dim point2 As New Point(t1Wp, 75 * ii)

            e.Graphics.DrawLine(grayPen1, point1, point2)
        Next ii

    End Sub


    Protected Overrides Sub OnPaint(ByVal e As System.Windows.Forms.PaintEventArgs)

        e.Graphics.SmoothingMode = SmoothingMode.HighQuality

        'routine to draw the grid
        DrawGrids(e)

        'routine to draw the curve
        DrawCurve(e)

    End Sub

    Private Sub DrawCurve(ByVal e As PaintEventArgs)

        e.Graphics.DrawLine(greenPen2, xGR1, yGR1, xGR2, yGR2)

        'stores 
        xGR1 = xGR2
        yGR1 = yGR2

        'clear screen after line reaches screen bottom
        If i >= (scrH / 3.75) - 1 Then
            i = 1

            ClearForm(e)

            yGR1 = 0

        End If
    End Sub

    Private Sub ClearForm(ByVal e As PaintEventArgs)
        'To clear the screen before continuing drawing
        e.Graphics.Clear(Color.Black)
    End Sub

    Private Sub tmrClock_Tick(ByVal sender As System.Object,
     ByVal e As System.EventArgs) Handles tmrClock.Tick

        'increment variable i to compute screen location of next point
        i = i + 1

        'computes screen location (x,y) of next point 
        xGR2 = xo1p + t1Wp * (GR(i) - GRmin) / (GRmax - GRmin)
        yGR2 = (DEPT(i) - DepthFirst) * 25

        Refresh()
    End Sub

End Class

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