Note that this code uses two transformations to make the PictureBox's coordinates increase from bottom to top.

Private Sub tmrGraph_Tick(ByVal sender As System.Object, _
    ByVal e As System.EventArgs) Handles tmrGraph.Tick
    ' Move the old data over.
    Dim bm As New Bitmap(picGraph.Width, picGraph.Height)
    Dim gr As Graphics = Graphics.FromImage(bm)
    gr.DrawImage(picGraph.Image, -m_Dx, 0)

    ' Erase the right edge and draw guide lines.
    gr.ScaleTransform(1, -101 / picGraph.Height)
    gr.TranslateTransform(0, -101)
    For i As Integer = 20 To 80 Step 20
        gr.DrawLine(Pens.Red, picGraph.Width - m_Dx, i, _
            picGraph.Width, i)
    Next i

    ' Plot the new value.
    Dim new_value As Integer = NewValue()
    gr.DrawLine(Pens.Black, _
        picGraph.Width - 1 - m_Dx, m_OldValue, _
        picGraph.Width - 1, new_value)
    m_OldValue = new_value

    ' Display the results.
    picGraph.Image = bm
End Sub