Subtotals in a datagrid

[Mark]

Is there a way to put a subtotal in a datagrid? For example, let's say I
have a datagrid of sales for a given year. No paging. I've used the
ItemDataBound event of the datagrid to add totals at the bottom. I'd like
to add quarterly subtotals in the MIDDLE of the grid on their own rows. The
data is sorted by default by date, so after the last March record, I want to
insert a subtotal.

Which event (ItemDataBound or ItemCreated) is the right one to use?
Suggestions on how to get started? Will I be adding records to the
underlying dataset OR adding new rows to the datagrid?

Thanks!

Mark