Offset Week in Expression

[#1]

Hi there

Can anyone help me with this one, basically due to the way we calculate the years week numbers this year has a week 1 but last year did not.  The desire is to she week one TY but against Week 2 from last year numbers.

I cannot change the week on the source data and as a result have been working with set analysis.

On one instance where the user had to select a week i used the below :-

=sum({$Year = {$(=max(Year)-1)}, Week = {$(=getfieldselections(Week)+1)}>} sales)

However I am now trying to do the same offset but on a pivot chart where i have weeks in columns.

I tried to do a similar set analysis but with no joy .....

Avg({$<Year = {$(=max(Year)-1)}, Week = {$(=Week+1)} >} sales)