hierarchical faceting with Elasticsearch

I'm using elasticsearch and need to implement facet search for hierarchical object as follow:

• category 1 (10)
  • subcategory 1 (4)
  • subcategory 2 (6)
• category 2 (X)
  • ...

So I need to get facets for two related objects. Documentation says that it's possible to get such kind of facets for numeric value, but I need it for strings http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-facets-terms-stats-facet.html

Here is another interesting topic, unfortunately it's old: http://elasticsearch-users.115913.n3.nabble.com/Pivot-facets-td2981519.html

Does it possible with elastic search? If so, how can I do that?