Hi Paul,
excuse me but I speek only a little english.
I'm looking for the best response for you, by now I found only an "half" response.
I fear that you cannot calculate in this chart the number of customers with growthing sales count, or decreasing one.
You need another dimension, resulting of the customers' aggregation for sales growth or decrease.
You can view total sales for year in the "total" column of a pivot table.
Look at my doc!
I hope it is useful.
Bye
