Dim m_X As Single
Dim m_Drawing As Boolean

Private Sub Form_MouseDown(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    m_Drawing = True
    DrawMode = vbInvert
    DrawWidth = 4
    m_X = X
    Line (m_X, ScaleTop)-(m_X, ScaleHeight)
End Sub

Private Sub Form_MouseMove(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    If Not m_Drawing Then Exit Sub
    Line (m_X, ScaleTop)-(m_X, ScaleHeight)
    m_X = X
    Line (m_X, ScaleTop)-(m_X, ScaleHeight)
End Sub

Private Sub Form_MouseUp(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    m_Drawing = False
    Line (m_X, ScaleTop)-(m_X, ScaleHeight)
End Sub