Database Custom Fields
If you want a database custom field to be searchable, select one of the available searchers in the Search Template field for that custom field. When you add a new database custom field, the "Database Exact Text Searcher" value is selected for a search template by default.
If the custom field already exists, go to Administration > Issues > Custom fields, click Edit for the desired custom field, and choose one of the available values for Search Template.
Child Database Custom Fields
Configuring searchers for child database custom fields is done in the same manner as for the database custom fields.
If you want a child database custom field to be searchable, select one of the available searchers in Search Template field for that custom field. When you add a new database custom field, the "Database Child Exact Text Searcher" value is selected for a search template by default.
If the custom field already exists, go to Administration > Issues > Custom fields, click Edit for the desired custom field, and choose one of the available values for Search Template.
Custom fields that use Free Text Searcher (such as Text Field or Text Area) are not supported in Two Dimensional Filter Statistics gadget.
This is also the case for Database Free Text Searcher and Database Child Text Searcher, which are based on the standard Free Text Searcher in Jira.
For your field value to be displayed correctly in this gadget, use the Database Exact Text Searcher/Database Child Exact Text Searcher.
Performing a Re-Index
After you change the search templates for all custom fields that you want, perform a re-index in Jira for the search to work fine.
This is also recommended in Jira documentation and tracked in JRA-4641.
Indexing
From time to time, the re-index has errors if you installed the app via UPM. Please edit and save a custom field on the Administration page and then try again. This problem is described also in Installation via Atlassian Universal Plugin Manager.