Multiple groups in Excel 2002

Hello there!

Hopefully you might be able to help me with a problem I'm trying to solve.

I have a spreadsheet with many workbooks. Each workbook contains sale figures for all products for each product range over a period of time.
There is one workbook that totals the figures from other workbooks.

This spreadsheet is updated by another department on a weekly basis with new figures added to each workbook (products within each product range are also likely to increase).

Currently, the data in each work book is grouped by months (with a row summarising the month's figures). I can create graphs to visualise monthly performance, but also need to have a weekly performance graph/chart. Thus, the data also needs to be grouped by weeks.

I have tried creating subgroups for the weeks within the month groups, but it seems impossible (?!) to do when a week subgroup comes right in the middle of a month summary (e.g. Dec 2008 Summary comes in between Week 29 Dec 2008 - 04 Jan 2009).

I have attached an example spreadsheet in case my description was too convoluted!

Any suggestions would be absolutely superb, and please feel free to put it in layman's terms

Many thanks!