Timesheet and invoice

Timesheet and invoice

Hi,
I want to create an invoice per month, but display the details of hours spent in the project per week.
I only have the choice between group by project (and I will have the hours spent during the whole month), or the detail of every timesheet submitted (and as I submit one per day, I have way to many lines).
Is there a trick to group by week and by project/task ?
Thanks,
Matthieu