Automatically record a total

I have a number of different "groups" each with prospects, clients and ex-clients connected to them.  I would like to track the fluctuation in group numbers on a weekly basis.  Knack can currently give me the totals in each group, as well as the grand total, but only as things stand presently. 

How do I get Knack to automatically record the totals at the end of say every week into a new table so I am able to "look back into the past" and see the number fluctuations over the last year for example?

That's what I would like to do - have you been able to achieve it yet?