# BalanceBeam (/docs/charts/balance-beam)

BalanceBeam answers "which of two sides outweighs, and roughly by how much?" A beam tilts toward the heavier side; the
direction is instant and the angle grows with the imbalance, but it **saturates** — read it as direction plus rough
magnitude, not an exact ratio. The two weights are area-true (their area, not their width, is proportional to value).
For a precise comparison, reach for `PairedBars` or `Delta`; this is for the glance.

```tsx
<BalanceBeam
  data={[
    { label: "Inflow", value: 620 },
    { label: "Outflow", value: 480 },
  ]}
  label="values"
  title="Cash flow"
  width={80}
  height={30}
/>
```

## Install

```tsx
import { BalanceBeam } from "@microcharts/react/balance-beam";

<BalanceBeam data={[{ label: "Inflow", value: 620 }, { label: "Outflow", value: 480 }]} />
```

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 buy vs sell or in vs out read in a sentence, a pro vs con weight in a KPI card, or an A-vs-B pair
  where direction is the story.
- **Avoid for** — exact ratios (PairedBars / Delta), more than two items (MiniBar), or trends.


## Variants

```tsx
<BalanceBeam
  data={[{ label: "Inflow", value: 620 }, { label: "Outflow", value: 480 }]}
  shape="round"
  label="values"
/>
<BalanceBeam data={[{ label: "A", value: 500 }, { label: "B", value: 500 }]} />
```

`format` also takes `Intl.NumberFormatOptions` — with a `locale`, both the announced values and the in-chart numerals
(`label="values"`) follow that locale's own grouping and decimal marks.

```tsx
<BalanceBeam
  data={[{ label: "Inflow", value: 62000 }, { label: "Outflow", value: 48000 }]}
  label="values"
  format={{ maximumFractionDigits: 0 }}
  locale="de-DE"
/>
```

## Edge cases

```tsx
<BalanceBeam data={[{ label: "A", value: 500 }, { label: "B", value: 500 }]} />
```

```tsx
<BalanceBeam data={[{ label: "A", value: 0 }, { label: "B", value: 0 }]} />
```

Equal values level the beam and report **"balanced"** rather than picking an arbitrary heavier side. Two zeros are the
same case — the beam stays level and both weights shrink to nothing, which is honest: there is nothing to weigh.


## Why this default

Ratio mode is the default because most two-sided comparisons are share-of-whole questions, and square weights are the
default because circles under-read area at this size. The tilt saturates at `maxTilt` degrees — a deliberate honesty
choice: past a point, a steeper beam would imply a precision the eye can't extract, so the angle levels off. Direction
and rough magnitude always agree, and the two channels (tilt and weight area) never disagree. For same-scale rows that
should tilt comparably, `mode="difference"` scales the imbalance by a shared `domain`.

## Accessibility

The accessible name states both sides and the winner — **"Inflow 620 vs Outflow 480; Inflow heavier."** — or "…;
balanced." when they match. The interactive entry eases the beam to its new tilt, reveals a side's value on hover or
←/→, and announces when the heavier side flips.

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) | `[{label,value},{label,value}]` | Exactly two items. |
| `maxTilt` | `number` | Degrees at full saturation (default 12). |
| `shape` | `"square" \| "round"` | Weight shape (default square). |
| `mode` | `"ratio" \| "difference"` | ratio = share-of-whole; difference = absolute, scaled by domain. |
| `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).