چند سری: زیرتجمیع + ترنسفورم flatten

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      { "key": "2026-06-08T08:00:00", "levels": { "buckets": [{ "key": "info", "doc_count": 420 }, { "key": "warn", "doc_count": 64 }, { "key": "error", "doc_count": 11 }] } },
      { "key": "2026-06-08T08:30:00", "levels": { "buckets": [{ "key": "info", "doc_count": 465 }, { "key": "warn", "doc_count": 58 }, { "key": "error", "doc_count": 9 }] } },
      { "key": "2026-06-08T09:00:00", "levels": { "buckets": [{ "key": "info", "doc_count": 510 }, { "key": "warn", "doc_count": 92 }, { "key": "error", "doc_count": 35 }] } },
      { "key": "2026-06-08T09:30:00", "levels": { "buckets": [{ "key": "info", "doc_count": 488 }, { "key": "warn", "doc_count": 130 }, { "key": "error", "doc_count": 88 }] } },
      { "key": "2026-06-08T10:00:00", "levels": { "buckets": [{ "key": "info", "doc_count": 530 }, { "key": "warn", "doc_count": 75 }, { "key": "error", "doc_count": 22 }] } },
      { "key": "2026-06-08T10:30:00", "levels": { "buckets": [{ "key": "info", "doc_count": 555 }, { "key": "warn", "doc_count": 61 }, { "key": "error", "doc_count": 12 }] } }
    ]
  },
  "transform": [
    { "flatten": ["levels.buckets"], "as": ["level"] },
    { "calculate": "datum.level.key", "as": "niveau" },
    { "calculate": "datum.level.doc_count", "as": "count" }
  ],
  "mark": "area",
  "encoding": {
    "x": { "field": "key", "type": "temporal", "title": null },
    "y": { "field": "count", "type": "quantitative", "stack": "zero" },
    "color": { "field": "niveau", "type": "nominal" }
  }
}