# ChangePoint (/docs/charts/change-point)

ChangePoint answers "when did the behavior change level?". It shades alternating regimes, draws a per-regime mean, and
marks the break where one level became another — because a spike means nothing without the regime it broke. The detector
is a **documented heuristic, not statistics**; for production anomaly pipelines, pass your own break indices and the
chart becomes pure annotation.

```tsx
import { ChangePoint } from "@microcharts/react/change-point";

<ChangePoint
  data={[
    30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48,
    48, 48, 48, 48, 48, 48, 48, 48, 48, 48,
  ]}
  label="delta"
  title="Error rate"
/>
```

## Install

```tsx
import { ChangePoint } from "@microcharts/react/change-point";

<ChangePoint data={errors} label="delta" title="Error rate" />
```

Setup (package + stylesheet): [Quickstart](/docs/quickstart#set-up-with-an-ai-agent) or paste [`/agent-setup.md`](/agent-setup.md) into your agent.


## When to use it

- **Good for** — context for an anomaly, an error rate / latency / cost that stepped to a new level, or annotating a
  known deploy or incident (pass explicit `breaks`).
- **Avoid for** — a gradual trend (Sparkline) or a plain time series with no regime question.


## Variants

```tsx
<ChangePoint
  data={[
    30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48,
  ]}
  breaks={[14]}
/>
<ChangePoint data={[20, 21, 23, 24, 26, 27, 29, 30, 32, 33, 35, 36, 38, 39, 41, 42, 44, 45, 47, 48]} />
```

## Edge cases

```tsx
<ChangePoint
  data={[3000, 3000, 3000, 3000, 3000, 3000, 3000, 5200, 5200, 5200, 5200, 5200, 5200]}
  label="delta"
  locale="de-DE"
/>
```

```tsx
// n < 8: auto-detection never fires, even on an obvious jump —
// pass explicit breaks for series this short
<ChangePoint data={[10, 10, 10, 50, 50, 50]} />
```

```tsx
<ChangePoint data={[42]} />
```

With a `locale`, the summary and delta label format their numbers in that locale's own grouping and decimal marks.
Auto-detection (`breaks="auto"`) only runs at 8 or more points — a six-point series with an obvious jump still renders
as one flat, unbroken regime unless you pass `breaks` explicitly; this is deliberate, since a two-segment fit on that
few points is mostly noise. A single point has no line to draw and no break to find; `data={[]}` renders the frame with
the "no data" summary and no marks.


## Why this default

Context for every anomaly — the shading names the regime a value belongs to, and the break marks where the level truly
changed, so a reader sees "errors stepped up here" rather than "errors are high". The detector is a **labelled
heuristic**: a two-segment mean-shift found by binary segmentation, accepted only when the split both cuts the variance
enough and clears an effect-size threshold, so a constant series shows no break. It is a mean-shift test, not a trend
test: a steady ramp is still fitted as two regimes (the ramp variant above reports a shift around point 10), so pass
`breaks` explicitly when you already know the series is trending rather than stepping. For anomaly pipelines that
already know their breakpoints, `breaks={[…]}` turns the detection off and the chart becomes a faithful annotation.

## Accessibility

The accessible name states the shift, the break, the regime means, and what happened after — **"Level shifted up 60%
around point 14 (mean 30 → 48); stable since."** — or, with no detected shift, "No clear level shift across N points."
The interactive entry steps the points with ←/→ (value + regime) and cycles the breaks with Tab, each announcing its
mean shift.

The interactive entry follows the shared [interaction contract](/docs/accessibility#one-interaction-contract):
arrow keys rove between units on both axes, `Home` and `End` jump to the ends, and a click, tap, `Enter` or
`Space` selects a unit — pinning its readout so it survives blur, until you select it again or press `Escape`.
On touch, a tap pins and a drag scrubs.

## Props

| Prop | Type | Description |
| --- | --- | --- |
| `data` (required) | `number[]` | A single series. |
| `breaks` | `"auto" \| number[]` | Explicit indices override the heuristic entirely — the production path. |
| `maxItems` | `number` | Max detected breaks (1–3). More regimes stop being glanceable. |
| `means` | `boolean` | Per-regime mean hairlines (default true). |
| `label` | `"delta" \| "none"` | Signed % across the most recent break, in a gutter. |
| `animate` | `boolean` | (interactive) Opt-in entrance motion when the chart mounts client-side — add `import "@microcharts/react/motion"` once. Inert on the server, on hydrated server HTML, and under `prefers-reduced-motion`. |

Plus the shared grammar — `data`, `domain`, `color`, `title`, `summary`, `format` — and the layout props (`width`, `height`, `className`, `style`) that every chart accepts. Interactive entries also share `animate` and `live`, and — wherever a chart has more than one navigable unit — `onActive`, `onSelect`, `selectedIndex` and `defaultSelectedIndex`; and — wherever the chart shows a hover value — `readout`. See [the shared grammar](/docs/quickstart#the-shared-grammar).