Vincent,

This is a simple solution, but might work for you. Place an image over the chart such that the chart can't be seen (image could be a simple white background or even a message such as "Not enough data").

Then, place this code in the worksheet's Change event:

Private Sub Worksheet_Change(ByVal Target As Range)

Sheet1.Shapes("Image1").Visible = WorksheetFunction.CountA(Range("$A$2:$A$23")) < 13

End Sub