Using the comparison reporting module is awkward due to the following UI
bugs: 1. You can't edit a custom comparison item - once created you'd
need to create it again, call it the same thing and then save over it 2.
If you do edit a custom comparison as per above and it's already being
used within a report you'll need to remove that comparison column and
then add it back in - it doesn't update automatically 3. If you want to
delete a custom comparison you can't unless you're prepared to delete
a...