This page shows how column group headers can be added to any Interactive Grid. Interactive Reports allowed defining column groups but they were only used in single row view. Interactive Grid lets you define column groups and choose if they are used for just report headings, just for single row view or both (the default is both). Column group headings can be used to reorder columns just like column headings. Play around with column reordering and frozen columns to see how the group headings are split and joined.
Column groups are a great feature, very well implemented, great job!
One minor nit - The Page Designer doesn't offer a way to view which columns are included in which groups, you have to click on each column and look at the Layout > Group attribute. Region Attributes > Column Groups > Group Name simply shows the group name, it would been have been nice if the columns included in the group were listed here as well
A quick dictionary query helps here so this is not a big deal, just a minor annoyance
c.region_name,c.name,c.heading column_heading,g.heading group_heading
from apex_appl_page_ig_columns c
join apex_appl_page_ig_col_groups g on c.group_id = g.group_id
where c.application_id = <app id>
and c.page_id = <page id>
and c.region_name = <region name>