Time elapsed between two events with EQL (and then use that in a visualization)

[syunusic (Sacha Yunusic)]

I have this log that has three relevant fields:

• @timestamp
• EVENTNO (Event number)
• DEVICEID (Device ID)

I want to know how much time happens between the start and the end of something specific (start of that something: EVENTNO="300095", end of it: EVENTNO="300096"), so I created this EQL search:

GET /redbanc-proview/_eql/search
{
  "query": """
  sequence by DEVICEID with maxspan=3h
   [any where EVENTNO == "300095"]
   [any where EVENTNO == "300096"]
  """,
  "fields": [
    {
      "field": "@timestamp",
      "format": "epoch_millis"  
    }
  ]
}

As result I got what I want: When a certain event starts (that's the one with EVENTNO 300095) and when it ends (EVENTNO 300096), great.
Now, I need how much time is between two events.

I transformed @timestamp into epoch_millis hoping that helps in the next step: subtract both @timestamp. I know EQL has subtract, so I was thinking soething like:

  "runtime_mappings": {
    "amount_of_time": {
      "type": "time",
      "script": "subtract(emit(doc['@timestamp'].value.event2,emit(doc['@timestamp'].value.event1)"
    }
  },

Of course this runtime mapping is totally made up, but is to get the idea.
Any ideas on how to do it?
And then, supposing I get that time... how could I put that into a visualization?