The set analysis is only evaluated once per chart while I assume you want to calculate per dimension value (Week),i.e. today's week - 4 weeks back / last week - 5 week back.
You can probably use chart inter record functions to get what you want, e.g. using above(), below() etc., maybe like
=sum(Sales) - above(sum(Sales),0,4)
Hope this helps,
As a simple example, this really helps, and it works - thank you!
The next stage is that i need to map on a 2nd dimension, so that i can view the sales differential history for each customer. When i try to add the 2nd dimension, unfortunately this does not work. It works with a simple expression, but when i introduce the Above() into the expression, it does not load.
I have a feeling i may need to do this a different way.