title: Aggregation of aggregation in Kibana visualization
body: <p>I am trying to add some metrics on top of a set of aggregations in Kibana, however I do not know to do that. 
  For example, my data looks like:</p>
  
  <pre><code>{{"A": 12, "B": 15, "month": "July"  }, 
   {"A": 5, "B": 98,  "month": "August"},
   {"A": 54, "B": 21, "month": "July"  },
   {"A": 53, "B": 4,  "month": "August"},
   {"A": 15, "B": 24, "month": "August"},
   {"A": 45, "B": 33, "month": "July"  },
   {"A": 25, "B": 56, "month": "August"},
   {"A": 8, "B": 23,  "month": "August"},
   {"A": 7, "B": 78,  "month": "July"  }}
  </code></pre>
  
  <p>I want to draw a line graph to show SUM(A)/SUM(B) over months. So the Y axis is <code>SUM(A)/SUM(B) per month</code> in this example. This is kind of applying aggregation functions over aggregation. I know that we can have aggregation of aggregation in elastic search, but what about in Kibana Visualization? Is it possible to do that in current version of Kibana (6.3)? Thanks!   <a href="https://i.stack.imgur.com/TzwmM.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TzwmM.jpg" alt="enter image description here"></a></p>