"Line" means a "row"?
You can do this with a macro but not a formula.
Formulas can only return values.
Here is a macro that inserts a blank row at every change in value in column A
Sub InsertRow_At_Change()
'Sandy Mann July 1st, 2007
Dim LastRow As Long
Dim x As Long
LastRow = Cells(Rows.Count, 1).End(xlUp).Row
Application.ScreenUpdating = False
For x = LastRow To 3 Step -1
If Cells(x, 1).Value <> Cells(x - 1, 1).Value Then
If Cells(x, 1).Value <> "" Then
If Cells(x - 1, 1).Value <> "" Then
Cells(x, 1).EntireRow.Insert Shift:=xlDown
End If
End If
End If
Next x
Application.ScreenUpdating = True
End Sub
If not familiar with VBA and macros, see David McRitchie's site for more on
"getting started".
http://www.mvps.org/dmcritchie/excel/getstarted.htm
In the meantime..........
First...create a backup copy of your original workbook.
To create a General Module, hit ALT + F11 to open the Visual Basic Editor.
Hit CRTL + r to open Project Explorer.
Find your workbook/project and select it.
Right-click and Insert>Module. Paste the code in there. Save the
workbook and hit ALT + Q to return to your workbook.
Run or edit the macro by going to Tool>Macro>Macros.
You can also assign this macro to a button or a shortcut key combo.
Gord Dibben MS Excel MVP