[Heapmap] using histogram, sort by value not key

[Leaf-Lin]

Describe the feature:
In the following xy data, I have all the data to be the same value (4 in this case), just x and y at various places.
Using heapmap, I can plot x y chart with buckets to be histogram field x and y.

As can be seen from the image, x-axis are all sorted by its key, but y-axis are not sorted.
I guess it's sorted by value, in which case, all the value are the same, so the order turns out to be random.

Is it possible to add options to sort by key?

This is the request body:

{
  "aggs": {
    "2": {
      "histogram": {
        "field": "x",
        "interval": 2,
        "min_doc_count": 1
      },
      "aggs": {
        "3": {
          "histogram": {
            "field": "y",
            "interval": 5,
            "min_doc_count": 1
          },
          "aggs": {
            "1": {
              "min": {
                "field": "data"
              }
            }
          }
        }
      }
    }
  },
  "size": 0,
  "_source": {
    "excludes": []
  },
  "stored_fields": [
    "*"
  ],
  "script_fields": {},
  "docvalue_fields": [],
  "query": {
    "bool": {
      "must": [
        {
          "query_string": {
            "query": "data:\"4\"",
            "analyze_wildcard": true,
            "default_field": "*"
          }
        }
      ],
      "filter": [],
      "should": [],
      "must_not": []
    }
  }
}

I've played with advanced options to add JSON input with sort to no avail.

Describe a specific use case for the feature:

It would be useful to plot xy charts.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)