_type field not indexed

A question about elasticsearch 2.3 and kibana 4.5. Anytime I create a new index in elasticsearch and I add to to kibana, I discover that the _type field is not indexed.
Kibana shows a warning message saying that the _type field is not available for visualization end search.
However, when I use it to scope a query to a given type of document in my index (_type: mydoctype) it seems to work, but I discovered strange behaviors using it in Graph plugin.

I did not notice this behavior with older versions of the stack: I have a couple of older indexes in elasticsearch, where the _type field appears as indexed.
If I recreate those indexes with the same data and scripts I get the problem of the _type field not indexed.

Is there anything I need to do when creating the index and/or when indexing my data to have the _type field correctly created as indexed? Thanks in advance.