How to make concentric rings in a pie chart per index

[Frances_Buontempo]

https://www.elastic.co/guide/en/elasticsearch/reference/5.2/mapping-index-field.html shows how to query elasticsearch using
"terms": {
"field" : "_index"
}

but you can't select the "field" _index in the kibana front end as a "Term" for the split slices.

My data has another filed I wish to split the pie-chart sections on, which I can do for one index or another, but I can't see how to make concentric rings for both at once

[spalger (Spencer Alger)]

Unfortunately the field_stats API which Kibana uses to get the list of aggregatable fields does not include information about the _index field... With https://github.com/elastic/elasticsearch/pull/23007 merged though we will be moving away from field_stats and the new API does support the _index field, so this should be fixed soon! Track https://github.com/elastic/kibana/issues/11014 for details