AFAIK you have to redefine ``read_group`` to fit your needs.

Nicolas is right: to do that I have to change read_group. It is a big method with no hooks, so the solution for now has been to copy all of it in my module and add a few lines similar to those the handle dates in the original code.

Date handling in the original is somewhat similar: dates from the same month are grouped together, while I wanted to group together string fields that are equal ignoring case.

That has been refactored in trunk to be hookable.

Thanks!