Add scripted field aggregations into date histogram

[StephenGoodall]

Is it possible to add an aggregated value of a scripted field into a date histogram?
I'm trying to create a Vega chart and so far I can get it to graph the "doc_count" value of the date histogram.

What I would like is to get a sum of a scripted field that I include in the request, so I can see the sum of that value per day.

Currently my Vega query is:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "title": "Event counts from all indexes",
  "data": {
    "url": {
      "index": "_all",
      "body": {
        "query": {
          "bool": {
            "must": [
              {
                "range": {
                  "@timestamp": {
                    "format": "strict_date_optional_time",
                    "gte": "2020-02-10T05:03:41.544Z",
                    "lte": "2020-02-17T05:03:41.544Z"
                  }
                }
              }
            ],
            "filter": [
              {
                "bool": {
                  "filter": [
                    {
                      "bool": {
                        "should": [{"match": {"message": "SUCCESS"}}],
                        "minimum_should_match": 1
                      }
                    },
                    {
                      "bool": {
                        "filter": [
                          {
                            "bool": {
                              "should": [
                                {"match": {"message": "errors"}}
                              ],
                              "minimum_should_match": 1
                            }
                          },
                          {
                            "bool": {
                              "should": [
                                {
                                  "match": {
                                    "application": "search-indexer-api"
                                  }
                                }
                              ],
                              "minimum_should_match": 1
                            }
                          }
                        ]
                      }
                    }
                  ]
                }
              }
            ],
            "should": [],
            "must_not": []
          }
        },
        "aggs": {
          "time_buckets": {
            "date_histogram": {
              "field": "@timestamp",
              "interval": {"%autointerval%": true},
              "extended_bounds": {
                "min": {"%timefilter%": "min"},
                "max": {"%timefilter%": "max"}
              },
              "min_doc_count": 0
            }
          }
        },
        "script_fields": {
          "search-indexer_indexed_items": {
            "script": {
              "lang": "painless",
              "source": "String message = params['_source']['message'];int indexedLoc = message.indexOf('indexed:')+8;int errorsLoc = message.indexOf(', errors:')+9;int tookLoc = message.indexOf(', took:')+7;int messageLength = message.length();return message.substring(indexedLoc, errorsLoc-9);"
            }
          },
          "search-indexer_errors": {
            "script": {
              "lang": "painless",
              "source": "String message = params['_source']['message'];int indexedLoc = message.indexOf('indexed:')+8;int errorsLoc = message.indexOf(', errors:')+9;int tookLoc = message.indexOf(', took:')+7;int messageLength = message.length();return message.substring(errorsLoc, tookLoc-7);"
            }
          },
          "search-indexer_took": {
            "script": {
              "lang": "painless",
              "source": "String message = params['_source']['message'];int indexedLoc = message.indexOf('indexed:')+8;int errorsLoc = message.indexOf(', errors:')+9;int tookLoc = message.indexOf(', took:')+7;int messageLength = message.length();return message.substring(tookLoc, messageLength-1);"
            }
          }
        }
      }
    },
    "format": {"property": "aggregations.time_buckets.buckets"}
  },
  "mark": "line",
  "encoding": {
    "x": {"field": "key", "type": "temporal", "axis": {"title": false}},
    "y": {
      "field": "doc_count",
      "type": "quantitative",
      "axis": {"title": "Document count"}
    }
  }
}

I imagine I might need to move the painless bits into the aggs section instead maybe but I'm not too sure. Ultimately I'd like to put each of the 3 values in as a field, then I'd like to use 2 of them to calculate an average (search-indexer_indexed_items/search-indexer_took) per day, to track whether it is increasing over time.

Am I on the right track thinking to put the fields in the agg section?