For presentation purposes, it would be nice to be able to have groups of headings. In this case, the headings would be in two rows - the first with the group names that span over the headings in the group, and the second with the actual headings.

For example:

In terms of how to set this up, maybe just some separator in the column name would create a grouping. e.g. “Email Invite V1/Clicks” and “Email Invite V2/Signups” would be grouped into “Email Invite V2” with subheadings “Clicks” and “Signups”.