Graphics

You need a little bit of API. Use the DrawPoint function as shown below.

Private Declare Function GetDC Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function MoveToEx Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, lpPoint As POINTAPI) As Long
Private Declare Function LineTo Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long) As Long
Private Type POINTAPI
    x As Long
    y As Long
End Type

Private Sub DrawPoint(fromX, fromY, toX, toY)
    Dim hDC_Grid As Long
    Dim PT As POINTAPI
    
    hDC_Grid = GetDC(MSFlexGrid1.hwnd)
    MoveToEx hDC_Grid, fromX, fromY, PT
    LineTo hDC_Grid, toX, toY
End Sub

Private Sub Command1_Click()
    'Draw a line from (5,5) to (100,100)
    DrawPoint 5, 5, 100, 100
End Sub