X-axis order in Line chart gets messed up after adding sub-bucket aggregation

[milanage]

Kibana version: 6.3.x

Elasticsearch version: 6.3.x

Server OS version:

Browser version: FireFox 61.0.1

Browser OS version:

Original install method (e.g. download page, yum, from source, etc.): download page

Describe the bug:

Adding a terms aggregation sub-bucket (by split series) in a Line chart seems to break the original orderBy of X-axis.

See screenshot 1 where Y-axis is the count metrics, X-axis (score-rank) is ordered by desc. And it's working:

[screenshot 1]

However, after adding a terms aggregration sub-bucket (with "literature.keyword" as shown in the screenshot), the order doesn't work any more:

[screenshot 2]

Expected behavior:
X-axis order should keep the same.

Any additional context:
Elasticsearch response from inspector shows the buckets in ascending order:

Elasticsearch response body

{
  "took": 5,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": 37,
    "max_score": 0,
    "hits": []
  },
  "aggregations": {
    "2": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "3": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
              {
                "key": "文献1",
                "doc_count": 8
              },
              {
                "key": "文献3",
                "doc_count": 11
              },
              {
                "key": "文献5",
                "doc_count": 2
              }
            ]
          },
          "key": 1,
          "doc_count": 21
        },
        {
          "3": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
              {
                "key": "文献1",
                "doc_count": 3
              },
              {
                "key": "文献4",
                "doc_count": 1
              }
            ]
          },
          "key": 2,
          "doc_count": 4
        },
        {
          "3": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
              {
                "key": "文献1",
                "doc_count": 1
              },
              {
                "key": "文献2",
                "doc_count": 2
              }
            ]
          },
          "key": 3,
          "doc_count": 3
        },
        {
          "3": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
              {
                "key": "文献1",
                "doc_count": 3
              }
            ]
          },
          "key": 4,
          "doc_count": 3
        },
        {
          "3": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
              {
                "key": "文献4",
                "doc_count": 1
              }
            ]
          },
          "key": 5,
          "doc_count": 1
        },
        {
          "3": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
              {
                "key": "文献3",
                "doc_count": 1
              }
            ]
          },
          "key": 7,
          "doc_count": 1
        },
        {
          "3": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
              {
                "key": "文献3",
                "doc_count": 3
              }
            ]
          },
          "key": 8,
          "doc_count": 3
        }
      ]
    }
  },
  "status": 200
}

[timroes]

Duplicate of #17532