numéro ligne cellule

             If Y1 = 0 And Cells(m, n) <> "" And Cells(m, n) <> "a" Then
         Y1 = Cells(m, 1)
         X1 = Cells(m, n)
         End If
         
         If Y1 <> 0 And Y2 = 0 And Cells(m, n) <> "" And Cells(m, n) <> "a" Then
         Y2 = Cells(m, 1)
         X2 = Cells(m, n)
         traceligne X1, Y1, X2, Y2, coul
         For l = 'N°ligne de la cellule dont est issue(X1) To 'N°ligne(X2)
             If Cells(l, n) = "a" Then
             pointY = Cells(l, 1)
             pointX = (((X2 - X1) / (Y2 - Y1) * (pointY - Y1)) + X1)
             tracerclecoul pointX, pointY, coul
             End If
         Next l
         End If