The query and aggregation looks fine. For next steps I would start digging into the data. If you go to discover, for that time range, does the doc count look correct? If you check with a time range from april to december, are there > 0 documents? It might be worth checking most recent or all documents to see if there's a problem with the timestamp too.
If things still look strange, we can try and remove kibana from the equation and query elasticsearch directly:
curl -XGET "http://localhost:9200/transazioni/_search" -H 'Content-Type: application/json' -d'
{
"query": {
"range" : {
"@timestamp" : {
"gte" : 1451602800000,
"lte" : 1483225199999,
"format": "epoch_millis"
}
}
}
}'