We found in the Grimoire stats for Maniphest that the number of closed tickets per month up to 2014 was 0, which obviously is wrong, since Bugzilla tickets were migrated to Maniphest.
The problem seems to be due to the migration process. Apparently, tickets closed in Bugzilla were migrated to Maniphest as closed tickets, but with no closing event. And the trouble is that for calculating closed tickets per month, and the backlogs in the past, in Grimoire we use closing events, which have a date.
See for example these two, closed but without closing event:
Compared to these two (look for "closed this task as Resolved"):
I wonder if the missing closing events could be migrated to Maniphest. If that is not possible, we will look for some workaround in Grimoire, but the stats for before 2014 will probably render void.
Bugzilla had about 7500 duplicates, with this distribution over time: