[ML] Anomalies chart doesn't plot values for datafeeds comprising aggregations

[bczifra]

Kibana version: 7.6.1

Elasticsearch version: 7.6.1

Browser version: Chrome 81.0.4044.129 (Official Build) (64-bit)

Browser OS version: macOS High Sierra 10.13.6

Describe the bug:
When using datafeeds which contain aggregations, the anomalies chart doesn't plot values. Instead, it looks like this, with all values plotted at 0:

Steps to reproduce:
Mappings:

PUT activation
{
  "mappings" : {
      "properties" : {
        "@timestamp" : {
          "type" : "date"
        },
        "rate_plan_id": {
          "type": "keyword"
        }
      }
    }
}

PUT creation
{
  "mappings": {
    "properties": {
      "@timestamp": {
        "type": "date"
      },
      "rate_plan_id": {
        "type": "keyword"
      }
    }
  }
}

Job:

PUT _ml/anomaly_detectors/creation_activation_per_rate_plan
{
  "job_id": "creation_activation_per_rate_plan",
  "description": "",
  "groups": ["rate_plan_analysis"],
  "analysis_config": {
    "summary_count_field_name" : "doc_count",    
    "bucket_span": "15s",
    "detectors": [
      {
        "detector_description" : """mean("create_active_diff")""",
        "function" : "mean",
        "field_name" : "create_active_diff",
        "partition_field_name": "rate_plan",
        "detector_index" : 0
      }
    ],
    "influencers": [
      "rate_plan"
    ]
  },
  "data_description": {
    "time_field": "@timestamp"
  },
  "analysis_limits": {
    "model_memory_limit": "12MB"
  }
}

Datafeed:

PUT _ml/datafeeds/creation_activate_per_rate_plan
{
  "job_id": "creation_activation_per_rate_plan", 
  "indices": [
    "activation",
    "creation"
  ],
  "query_delay": "10000ms",
  "query": {
    "bool": {
      "must": [
        {
          "match_all": {}
        }
      ]
    }
  },
  "aggs": {
    "buckets": {
      "date_histogram": {
        "field": "@timestamp",
        "fixed_interval": "15s"
      },
      "aggs": {
        "@timestamp": {
          "max": {
            "field": "@timestamp"
          }
        },
        "rate_plan": {
          "terms": {
            "field": "rate_plan_id",
            "size": 10
          },
          "aggs": {
            "creations": {
              "filter": {
                "bool": {
                  "filter": {
                    "term": {
                      "_index": "creation"
                    }
                  }
                }
              }
            },
            "activations": {
              "filter": {
                "bool": {
                  "filter": {
                    "term": {
                      "_index": "activation"
                    }
                  }
                }
              }
            },
            "create_active_diff": {
              "bucket_script": {
                "buckets_path": {
                  "creations": "creations>_count",
                  "activations": "activations>_count"
                },
                "script": "params.creations - params.activations"
              }
            }
          }
        }
      }
    }
  }
}

Expected behavior:
The chart should either be disabled if we can't plot values, or values should be plotted correctly.

[qn895]

Closing via #81923 and #84594