In my chart, each month I have to add another set of data. Usually, I go to
Select Data, and edit the Series by Contol + select the newest cell. This
month (October), when I try to Control+Select, all the previously chosen
cells are deselected.

It's almost as if this method works for 9 entries, but when I try to do 10
(October being the 10th month), it doesn't allow me to add that many values.

How can I easily add my 10th month (and 11th & 12th) of data to a chart?