XML report design question: is it possible to sum across different groups?

 Dear all,

I want to define a template for AP Cash Requirement report to list invoices to be paid by vendor and also give a subtotal by vendor. However, the XML tree of this report is to first group by Due Date (G_Date) and then group by Vendor (G_Vendor), which means, the invoices for the same vendor could be under different G_Date group. Is it possible to subtotal by vendor regardless of which Due Date group the invoices belong to?

Thanks!