End Of Workweek Subtotal

I have a report with sales YTD. I need to split it by weeks (incremental YTD) so I can use the subtotals to create a graph showing how we’re getting close to our goal for the year. To be clearer, what I mean by incremental is creating a YTD by week column like shown below.

Week#SalesYTD
Week1$250,000
Week2$200,000$450,000
Week3$350,000$800,000

The report gives me daily invoices and the amount as shown below

Inv DateExt. Cost
4/1/2007$389.25
4/2/2007$5,266.83
4/5/2007$57.63
4/3/2007$164.15
4/3/2007$283.61
4/5/2007$32.12
4/4/2007$518.25
4/8/2007$2,309.80
4/8/2007$2,887.25
4/2/2007$150.90
4/8/2007$2,046.75