"Not plotted" option / blank cells in graph
 

I have a similar problem to "blank cells in graph" already posted by Kirsty,
ie. 'Plot empty cells as: Not plotted' option is greyed out.
The cells are empty, (no formula).
My intention is to extend the formula from week to week (every time the
chart will be updated.
I'm using a Stacked Area chart - maybe that's the problem?
ie. it's not possible to have a Stack Area chart to "stop"?
Thanks in advance....