Grouped by ITEM, not by window. A window is how the archive stores; an item with a series is how the question is asked. Capped at 5000 series points and a 90-day span, both enforced in the database. When the cap is hit, truncated is true and the series is incomplete — narrow the interval rather than drawing conclusions from a partial series.

An unknown niche is NOT a 404: nothing validates the name, so a typo returns items: [] with points: 0 — AND still spends a call, because the quota counter increments before the archive is read. Resolve niche names at /api/v1/nichos first.

{
  "items": [
    {
      "platform": "hackernews"
    }
  ]
}