After adding a new nFeed field to your system, you must perform two additional steps to ensure that the nFeed field can be reported on correctly from Dataplane:
Reindex JIRA. This step is not just required by Dataplane, but it is also required to be able to see your data in the rest of JIRA. (If your version of JIRA supports it, you may be do this as a background reindex operation. Indexing only the impacted project is also fine. In general, if you can see the field as a column in the JIRA Issue Navigator for all of the relevant issues, Dataplane will be able to work on the field.)
If the nFeed field was newly added and it does not appear in the Segment By control in Dataplane, go to Dataplane Reports » Administration and click Sync Index to refresh the list of accessible custom fields. In most situations, a Dataplane "Sync Index" operation only takes a few seconds and it does not cause any downtime for Dataplane users.