This week and same week last year

Hi All, 

I've tried a couple of methods to get this to work, but I can't quite make it gel. I have a Table that has the following columns:

Amount, Payment Date, New Subscription and Category.

I also have a Calendar dimension that is joined on Payment Date, in the Calendar Dimension and in that Table I have a WeekNum column (along with other date formats). I have 3 graphs:

1: Showing the Dollar Amount of New Subscriptions, paid for in the last 7 days, by category

2: Showing the Dollar amount of Renewal Subscription (e.g. where the new Subscription flag isn't set), paid for in the last 7 days, by category

3: Showing Total Dollar Amount, paid for in the last 7 days

Currently, for each graph I just have a relative date filter - however I've been asked to add to each of these a secondary value for the same time last year. So this means I'll have to remove the relative date filter and probably tweak my current measures and create a new measure for the same period last year - however I've not had any success doing this,