The split_by corollary to [GroupRollupMode]. Flat (the default,
matching this crate's historical behavior) emits only full-depth split
combinations as columns; Rollup additionally emits grand-total and
subtotal column groups in "totals before" order. There is no Total
variant - an empty split_by already expresses a single grand-total
column group.
The
split_bycorollary to [GroupRollupMode].Flat(the default, matching this crate's historical behavior) emits only full-depth split combinations as columns;Rollupadditionally emits grand-total and subtotal column groups in "totals before" order. There is noTotalvariant - an emptysplit_byalready expresses a single grand-total column group.