# QueueDepth (/docs/charts/queue-depth)

QueueDepth answers "is the backlog draining or growing?". It draws the queue **depth** as a zero-anchored area — a
stock, not a rate — against a dashed **capacity** hairline. Wherever the depth runs above capacity, the top edge is
re-stroked in the negative ink, so a breach is a change in _shape_ as well as color. The endpoint carries the current
value and a trend glyph (▴/▾) fit over the last quarter of the window.

```tsx
<QueueDepth
  data={[42, 55, 70, 88, 96, 120, 150, 182, 214]}
  capacity={100}
  title="Support queue"
  width={260}
  height={30}
/>
```

## Install

```tsx
import { QueueDepth } from "@microcharts/react/queue-depth";

<QueueDepth
  data={[42, 55, 70, 88, 96, 120, 150, 182, 214]}
  capacity={100}
  title="Support queue"
/>
```

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** — a support-queue backlog in a KPI card, a work-in-progress stock against its WIP limit, will-it-drain in
  a tab header.
- **Avoid for** — a rate rather than a stock (Sparkline) or a single count (Delta).


## Variants

```tsx
const backlog = [42, 55, 70, 88, 96, 120, 150, 182, 214];

<QueueDepth data={[214, 190, 150, 120, 96, 70, 48]} capacity={100} />
<QueueDepth data={backlog} />
<QueueDepth data={backlog} capacity={100} label="none" />
```

```tsx
<QueueDepth
  data={[420, 560, 720, 880, 1040, 1280, 1560, 1880, 2140]}
  capacity={1000}
  locale="de-DE"
/>
```

With a `locale`, both the endpoint label and the accessible summary's numbers follow that locale's own grouping — the
summary reads **"2.140 queued, 2,1× capacity, growing over the last quarter."** in German, not "2,140 queued, 2.1×
capacity".

## Edge cases

```tsx
// with no capacity the area still draws, but there's nothing to breach —
// the summary reports depth and trend without a capacity clause
<QueueDepth data={[42, 55, 70, 88, 96, 120, 150, 182, 214]} />
```

```tsx
<QueueDepth data={[90, 70, 50, 30]} capacity={100} />
```

```tsx
// null / NaN / ±Infinity break the area into separate runs — a measured
// gap, never interpolated across
<QueueDepth data={[10, null, 40, null, 80, 120]} capacity={100} />
```

With no `capacity`, the area still draws and the summary reads **"214 queued, growing over the last quarter."** — no
capacity clause, because there's nothing to compare against. A queue that stays under capacity reports **"30 queued,
within capacity, draining over the last quarter."** and never re-strokes the edge. Gaps (`null`/`NaN`/`±Infinity`) split
the area into separate runs rather than drawing a line across missing periods.


## Why this default

The capacity hairline plus the negative re-stroke, because "are we keeping up?" is a threshold question — a breach
should read instantly and survive greyscale, so it changes the edge's _shape_, not only its color. The trend glyph is
deliberately low-precision: it's fit over the last quarter of the window as a direction cue, and the summary states that
window in words ("over the last quarter") rather than implying a forecast the data can't support.

## Accessibility

The accessible name states the current depth, the capacity relationship, and the trend — **"214 queued, 2.1× capacity,
growing over the last quarter."**. When the depth stays under capacity the middle clause becomes "within capacity"; with
no `capacity` it drops entirely. The interactive entry steps the periods, announcing each depth and whether it's above
capacity (**"t8: 214 queued, above capacity."**).

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[]` | Backlog depth per period (≥ 0). null / NaN / ±Infinity are gaps. |
| `capacity` | `number` | Steady-state capacity: a dashed hairline; spans above it re-stroke negative. |
| `label` | `"last" \| "none"` | Endpoint value + trend glyph (▴/▾), default 'last', or nothing. |
| `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).