# HeatCell (/docs/charts/heat-cell)

HeatCell answers "how intense is this one value against a known scale?". It is the single-cell building block for grids
you lay out yourself — a matrix of tenants × hours, a custom calendar, a density chip in a sentence. Color is quantized
into discrete steps: continuous opacity would fake precision a 12-px cell can't deliver.

```tsx
<span className="inline-flex items-center gap-1.5">
  {[12, 35, 58, 79, 96].map((v) => (
    <HeatCell key={v} value={v} domain={[0, 100]} title={`Load ${v}`} style={{ width: 18, height: 18 }} />
  ))}
</span>
```

## Install

```tsx
import { HeatCell } from "@microcharts/react/heat-cell";

<HeatCell value={42} domain={[0, 100]} title="Load" />
```

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** — table-cell matrices, custom grids, intensity chips beside labels.
- **Avoid for** — precise comparison (MiniBar, DotPlot) or time series (HeatStrip, ActivityGrid).

## The shared-domain rule

A lone cell has no data to auto-scale from, so `domain` defaults to `[0, 1]` — pass your grid's real scale. **Every cell
in one grid must share one domain**: per-cell auto-scaling would make the brightest hour of a quiet tenant look like the
brightest hour of a loud one.

```tsx
// one domain, computed once, shared by every cell
const domain: [number, number] = [0, Math.max(...allValues)];
rows.map((r) => r.hours.map((v) => <HeatCell value={v} domain={domain} />));
```


## Variants

```tsx
<HeatCell value={70} domain={[0, 100]} shape="square" />
<HeatCell value={70} domain={[0, 100]} shape="round" />
<HeatCell value={70} domain={[0, 100]} shape="dot" />
```

```tsx
<HeatCell value={8} domain={[0, 9]} label="value" />
```

## Edge cases

```tsx
<HeatCell value={NaN} domain={[0, 100]} />
```

```tsx
<HeatCell value={-20} domain={[0, 100]} />
<HeatCell value={140} domain={[0, 100]} />
```


## Why this default

Five steps is the most a reader reliably distinguishes at cell size — the same ramp ActivityGrid uses, so intensity
means one thing across the library. Values outside the domain clamp to the end steps (documented, never silently
rescaled), and a zero-width domain renders the single mid step with a dev warning.

## Accessibility

The accessible name carries the value _and_ its calibration — the shape cells above read **"70 — level 4 of 5."** — so
the color scale is never the only channel. Non-finite input renders a designed empty track and says **"No data."** The
interactive entry reveals the same reading on hover/focus, with ActivityGrid announcement parity.

This chart is a single unit, so there is nothing to rove between: a click, tap, `Enter` or `Space` selects it
and fires `onSelect`, and no selection stays pinned. That is the scalar half of the shared
[interaction contract](/docs/accessibility#one-interaction-contract).

## Props

| Prop | Type | Description |
| --- | --- | --- |
| `value` (required) | `number` | The value to calibrate. |
| `domain` | `[number, number]` | Calibration scale — defaults to [0, 1]; every cell in a grid must share one. |
| `steps` | `number` | Discrete perceptual steps (default 5, shared with ActivityGrid). |
| `shape` | `"square" \| "round" \| "dot"` | Shared cell vocabulary. |
| `label` | `"value" \| "none"` | Centered number when the cell doubles as a chip. |
| `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).