Waterfall chart

"use client"

import * as React from "react"
import { Bar, CartesianGrid, Cell, LabelList, BarChart as RechartsBarChart, XAxis, YAxis } from "recharts"

import { cn } from "@/lib/utils"
import { formatNumber } from "@/lib/format"
import {
  ChartContainer,
  ChartTooltip,
  ChartTooltipContent,
  type ChartConfig,
} from "@/components/ui/chart"
import { AXIS_TICK, axisProps, BAR_RADIUS, chartDefaults, GRID_PROPS } from "@/components/ui/chart-core"
import { niceDomain } from "@/components/ui/chart-scale"

const MAX_BAR_SIZE = 40

export type WaterfallMovement = {
  label: string
  value: number
  /** Draws the running total so far from zero, instead of the movement itself. */
  isTotal?: boolean
}

export type WaterfallRow = {
  label: string
  base: number
  delta: number
  fill: "success" | "danger" | "primary"
  total: number
}

/**
 * Each movement as a floating bar: `base` is where it starts on the scale and
 * `delta` how tall it is, so a rise and the fall that undoes it both draw
 * upward from different places. `total` is the running total after the row,
 * which is what the tooltip reads out.
 *
 * A row marked `isTotal` is a subtotal rather than a movement: it draws from
 * zero to the running total as it stands, in the neutral colour, and leaves
 * that total untouched — so a closing row can carry any `value` at all,
 * including none. Everything else is a movement, including an opening balance,
 * which reads as the rise it is.
 */
export function toWaterfallData(data: WaterfallMovement[]): WaterfallRow[] {
  let running = 0
  return data.map(({ label, value, isTotal }) => {
    if (isTotal) {
      return {
        label,
        base: Math.min(0, running),
        delta: Math.abs(running),
        fill: "primary" as const,
        total: running,
      }
    }
    const next = running + value
    const row = {
      label,
      base: Math.min(running, next),
      delta: Math.abs(value),
      fill: value < 0 ? ("danger" as const) : ("success" as const),
      total: next,
    }
    running = next
    return row
  })
}

const FILL: Record<WaterfallRow["fill"], string> = {
  success: "var(--success)",
  danger: "var(--danger)",
  primary: "var(--primary)",
}

const DEFAULT_FORMAT = (value: number) => formatNumber(value, { maximumFractionDigits: 0 })

export type WaterfallChartProps = React.ComponentProps<"div"> & {
  data: WaterfallMovement[]
  valueFormatter?: (n: number) => string
  height?: number
  showGrid?: boolean
}

function WaterfallChart({
  className,
  data,
  valueFormatter = DEFAULT_FORMAT,
  height = chartDefaults.height,
  showGrid = chartDefaults.showGrid,
  ...props
}: WaterfallChartProps) {
  const rows = React.useMemo(() => toWaterfallData(data), [data])
  // `fill` is renamed on the way into recharts, which paints a bar from a
  // `fill` on its own datum: a <Cell> still wins on the delta bar, but the
  // run-up bar has no Cell, so the word "success" beat its fill="transparent"
  // and reached the captions as a colour. The caption is the signed movement,
  // which the bar's height alone cannot say: a fall and the rise that matches
  // it are the same height.
  const plotted = rows.map(({ fill, ...row }, i) => ({
    ...row,
    paint: FILL[fill],
    caption: data[i].isTotal
      ? valueFormatter(row.total)
      : `${data[i].value < 0 ? "−" : "+"}${valueFormatter(Math.abs(data[i].value))}`,
  }))

  // A printed scale: the ticks land on 1/2/2.5/5 steps over the tallest column
  // a row reaches, so the reader can place a bar between two gridlines without
  // arithmetic.
  const scale = niceDomain(
    Math.min(0, ...rows.map((row) => row.base)),
    Math.max(0, ...rows.map((row) => row.base + row.delta))
  )
  const config: ChartConfig = { delta: { label: "Movement" } }
  const summary =
    rows.length === 0
      ? "Waterfall chart with no movements."
      : `Waterfall chart of ${rows.length} movements ending at ${valueFormatter(rows[rows.length - 1].total)}.`

  return (
    <div data-slot="waterfall-chart" className={cn("w-full", className)} {...props}>
      <ChartContainer
        config={config}
        role="img"
        aria-label={summary}
        className="aspect-auto w-full"
        style={{ height }}
      >
        <RechartsBarChart
          // Named once, by the container's role="img" and its summary; recharts'
          // keyboard layer would add an unnamed role="application" tab stop inside it.
          accessibilityLayer={false}
          data={plotted} margin={{ top: 20, right: 12, bottom: 0, left: 12 }}>
          {showGrid ? <CartesianGrid vertical={false} {...GRID_PROPS} /> : null}

          <XAxis tick={AXIS_TICK} dataKey="label" {...axisProps} minTickGap={4} interval={0} />
          <YAxis
            tick={AXIS_TICK}
            {...axisProps}
            width="auto"
            domain={scale.domain}
            ticks={scale.ticks}
            tickFormatter={(value) => valueFormatter(Number(value))}
          />

          <ChartTooltip
            content={
              <ChartTooltipContent
                // The primitive prints raw numbers, and a floating bar's own
                // height is only half the story, so the row is rebuilt to show
                // the movement beside the total it leaves behind.
                formatter={(_value, _name, item) => {
                  const row = item.payload as Omit<WaterfallRow, "fill"> & { caption: string }
                  return (
                    <div className="flex flex-1 items-center justify-between gap-3 leading-none">
                      <span className="text-muted-foreground">{row.caption}</span>
                      <span className="font-medium tabular-nums">{valueFormatter(row.total)}</span>
                    </div>
                  )
                }}
              />
            }
          />

          {/* The lower half of every stack is the run-up to the bar, painted in
              nothing — it is what floats the movement off the baseline. */}
          <Bar
            dataKey="base"
            stackId="waterfall"
            fill="transparent"
            legendType="none"
            tooltipType="none"
            maxBarSize={MAX_BAR_SIZE}
            isAnimationActive={false}
          />
          {/* Neither bar animates: they float, and growing each one from the
              baseline draws a different chart on the way up. */}
          <Bar
            dataKey="delta"
            stackId="waterfall"
            radius={[BAR_RADIUS, BAR_RADIUS, 0, 0]}
            maxBarSize={MAX_BAR_SIZE}
            isAnimationActive={false}
          >
            {plotted.map((row, i) => (
              <Cell key={`${row.label}-${i}`} fill={row.paint} />
            ))}
            <LabelList
              dataKey="caption"
              position="top"
              fontSize={11}
              className="fill-muted-foreground tabular-nums"
            />
          </Bar>
        </RechartsBarChart>
      </ChartContainer>

      <ul data-slot="waterfall-chart-data" aria-label={summary} className="sr-only">
        {plotted.map((row, i) => (
          <li key={`${row.label}-${i}`}>{`${row.label}: ${row.caption}, running total ${valueFormatter(row.total)}`}</li>
        ))}
      </ul>
    </div>
  )
}

export { WaterfallChart }

import * as React from "react"

import { formatNumber } from "@/lib/format"
import type { ChartConfig } from "@/components/ui/chart"
import { isChartToken, type ChartToken } from "@/components/ui/percentage-bar"

/** One point of a chart: the index value plus one number per series key. */
export type ChartDatum = Record<string, string | number | null | undefined>

/** One plotted measure — which key to read, what to call it, what to paint it. */
export type ChartSeries = {
  key: string
  label: string
  /** A chart token, or any CSS colour for a brand hue. Defaults to the palette in order. */
  color?: ChartToken | string
  /**
   * The key holding the same measure over the period before this one. Drawn as
   * a dashed ghost behind the series and printed in its tooltip row as a delta.
   */
  compareKey?: string
  /**
   * The colour the ghost is drawn in: a chart token or any CSS colour. Left
   * out, the ghost wears the series' own colour; `var(--chart-neutral)` keeps
   * the period before out of the palette, grey under every preset — including
   * one whose first slot follows a coloured accent.
   */
  compareColor?: ChartToken | string
}

/** The props every cartesian chart in the set accepts. */
export type CommonChartProps = React.ComponentProps<"div"> & {
  data: ChartDatum[]
  /** The key holding each point's category — the month, the day, the source. */
  index: string
  series: ChartSeries[]
  /** Formats every number the chart prints: axis ticks, tooltips, and the text summary. */
  valueFormatter?: (n: number) => string
  /** Formats every index label, e.g. an ISO date into "Sep 4". */
  indexFormatter?: (v: string | number) => string
  /** Plot height in pixels, axis band included. */
  height?: number
  showLegend?: boolean
  showGrid?: boolean
  showXAxis?: boolean
  showYAxis?: boolean
  showTooltip?: boolean
  /** Where the series are named: beside their last point, under the plot, or nowhere. */
  legend?: ChartLegendPlacement
  /** Goal lines, event markers and bands drawn over the plot and read out as text. */
  annotations?: ChartAnnotation[]
  /** Draws each series' `compareKey` as a dashed ghost behind it, in the series' `compareColor` if it names one, its own colour if not. */
  compare?: boolean
  className?: string
}

// The palette wraps rather than inventing a ninth hue: past eight series the
// colours stop being distinguishable, so fold the tail into an "Other" series.
const PALETTE_SIZE = 8

/**
 * A series' colour: a chart token becomes its CSS variable, any other string is
 * taken as a raw CSS colour, and a series that names none takes the next slot in
 * the palette, wrapping past the eighth.
 */
export function seriesColor(series: ChartSeries, i: number): string {
  const color = series.color
  if (color === undefined) return `var(--chart-${(i % PALETTE_SIZE) + 1})`
  return isChartToken(color) ? `var(--${color})` : color
}

/**
 * The ChartConfig the chart primitive needs — it turns each entry into a
 * `--color-<key>` custom property that recharts marks reference.
 */
export function buildChartConfig(series: ChartSeries[]): ChartConfig {
  // fromEntries defines own properties, so a series keyed "__proto__" lands as
  // data instead of reassigning the config object's prototype.
  return Object.fromEntries(
    series.map((entry, i) => [entry.key, { label: entry.label, color: seriesColor(entry, i) }])
  )
}

/**
 * The shared starting point: a 280px plot with both axes, a grid, direct labels
 * at the end of each series, and a tooltip. The value axis is on — a chart
 * without a printed scale is a shape, not a measurement — and the charts turn it
 * off themselves when the plot is too small to carry one (`fitsYAxis`).
 */
export const chartDefaults = {
  height: 280,
  showLegend: true,
  showGrid: true,
  showXAxis: true,
  showYAxis: true,
  showTooltip: true,
  legend: "inline-end",
} as const

/**
 * A drawn line: 2px with round caps and joins reads as a stroke at any size
 * and holds its own on a white sheet, where 1.5px thinned to a hair on a
 * high-density screen.
 */
export const STROKE_WIDTH = 2

/**
 * The gridline: the grid token, dashed, so a rule under the data reads as a
 * guide rather than as a border. Every cartesian chart spreads this onto its
 * CartesianGrid.
 */
export const GRID_PROPS = { stroke: "var(--chart-grid)", strokeDasharray: "3 3" } as const

/**
 * The dot under the pointer: 3.5px of the series' own colour, ringed in the
 * surface it sits on so it stays legible where two lines cross.
 * --chart-surface falls back to the card a chart normally lives on; set it on
 * any ancestor when the chart sits on another plane.
 */
export const ACTIVE_DOT = { r: 3.5, strokeWidth: 2, stroke: "var(--chart-surface, var(--card))" } as const

/**
 * The corner a column turns, and the gap between two segments of one stack. A
 * stack is rounded as one shape — the top corners on the topmost segment, the
 * bottom corners on the lowest — so a bar reads as a printed block rather than
 * a pile of separately rounded tiles.
 */
export const BAR_RADIUS = 6
export const BAR_GAP = 1

/** Axis chrome: no rule and no tick marks, so only the labels carry the scale. */
export const axisProps = { tickLine: false, axisLine: false, tickMargin: 8, fontSize: 12 } as const

/**
 * The paint every axis label takes: --chart-axis, the token §3.8 names for the
 * printed scale. recharts writes fill="#666" onto its own
 * tick text, and the primitive's `.recharts-cartesian-axis-tick text` rule does
 * not reach it — recharts 3 nests labels under
 * `.recharts-cartesian-axis-tick-labels` instead — so #666 survived on the card
 * at 3.16:1 in dark. Passing the fill as a tick prop puts it on the element
 * itself, where nothing has to match a selector.
 */
export const AXIS_TICK = { fill: "var(--chart-axis)" } as const

/** Chart numbers fall back to thousands-separated values when no valueFormatter is given. */
export function defaultValueFormatter(value: number): string {
  return formatNumber(value)
}

/** Index labels print as they arrive unless the chart is given an indexFormatter. */
export function defaultIndexFormatter(value: string | number): string {
  return String(value)
}

/** What the text helpers need to turn a chart's data back into words. */
export type ChartTextOptions = {
  data: ChartDatum[]
  index: string
  series: ChartSeries[]
  valueFormatter?: (n: number) => string
  indexFormatter?: (v: string | number) => string
}

function readIndex(datum: ChartDatum, index: string, format: (v: string | number) => string) {
  const raw = datum[index]
  return typeof raw === "string" || typeof raw === "number" ? format(raw) : ""
}

function readValue(raw: ChartDatum[string], format: (n: number) => string) {
  return typeof raw === "number" && Number.isFinite(raw) ? format(raw) : "no data"
}

function joinLabels(labels: string[]): string {
  if (labels.length < 2) return labels.join("")
  return `${labels.slice(0, -1).join(", ")} and ${labels[labels.length - 1]}`
}

/**
 * The plotted data as text, one row per point. Every chart renders this into a
 * visually hidden list, so the numbers are readable without pointing at a tooltip.
 */
export function chartRows(options: ChartTextOptions): { label: string; readings: string }[] {
  const {
    data,
    index,
    series,
    valueFormatter = defaultValueFormatter,
    indexFormatter = defaultIndexFormatter,
  } = options

  return data.map((datum) => ({
    label: readIndex(datum, index, indexFormatter),
    readings: series
      .map((entry) => `${entry.label} ${readValue(datum[entry.key], valueFormatter)}`)
      .join(", "),
  }))
}

/**
 * The heading for a tooltip: the point's own index value, read straight off the
 * datum. The primitive resolves its label through the config, which only works
 * when the index is a string — reading the datum keeps numeric indexes intact.
 */
export function tooltipIndexLabel(
  payload: ReadonlyArray<{ payload?: unknown }> | undefined,
  index: string,
  format: (v: string | number) => string = defaultIndexFormatter
): string {
  const datum = payload?.[0]?.payload
  const raw = datum && typeof datum === "object" ? (datum as ChartDatum)[index] : undefined
  return typeof raw === "string" || typeof raw === "number" ? format(raw) : ""
}

/** A one-sentence description of what a chart plots, used as its accessible name. */
export function chartSummary(kind: string, options: ChartTextOptions): string {
  const { data, index, series, indexFormatter = defaultIndexFormatter } = options
  if (series.length === 0) return `${kind} with no series.`

  const labels = joinLabels(series.map((entry) => entry.label))
  if (data.length === 0) return `${kind} of ${labels} by ${index}. No data.`

  const first = readIndex(data[0], index, indexFormatter)
  const last = readIndex(data[data.length - 1], index, indexFormatter)
  const count = `${data.length} point${data.length === 1 ? "" : "s"}`
  return `${kind} of ${labels} by ${index}, ${count} from ${first} to ${last}.`
}

/* -------------------------------------------------------------------------- */
/* Annotations                                                                 */
/* -------------------------------------------------------------------------- */

/** Where a chart names its series. */
export type ChartLegendPlacement = "inline-end" | "bottom" | "none"

/** The meanings an annotation can carry; each resolves to a token, never a literal. */
export type ChartAnnotationTone = "neutral" | "brand" | "positive" | "negative" | "warning" | "info"

export type ChartAnnotation =
  /** A threshold across the plot: a goal, a limit, an included allowance. */
  | { kind: "line"; axis?: "x" | "y"; value: number | string; label: string; tone?: ChartAnnotationTone }
  /** A moment on the index axis: a launch, a deploy, an incident. */
  | { kind: "event"; x: number | string; label: string; tone?: ChartAnnotationTone; href?: string }
  /**
   * A stretch of the index axis: a freeze, an outage, a campaign. The label
   * reads from the band's start; `align: "end"` hangs it from the band's end
   * instead, for a band that runs to the edge of the plot — a 59px word over
   * a 30px band at the right edge otherwise runs out of the plot.
   */
  | {
      kind: "band"
      from: number | string
      to: number | string
      label: string
      tone?: ChartAnnotationTone
      align?: "start" | "end"
    }

const ANNOTATION_PAINT: Record<ChartAnnotationTone, string> = {
  neutral: "var(--faint-foreground)",
  brand: "var(--brand)",
  positive: "var(--chart-positive)",
  negative: "var(--chart-negative)",
  warning: "var(--warning)",
  info: "var(--info)",
}

/**
 * The paint a *meaning* takes, as opposed to a category.
 *
 * A series that is coded by status — 200/429/500, up/down, paid/overdue — is
 * not one of eight interchangeable hues: it has to resolve to the semantic
 * tokens, or a reader learns the wrong colour for "failed" on one page and
 * carries it to the next. Categories keep the palette; meanings come from here.
 */
export const CHART_TONES = {
  positive: "var(--chart-positive)",
  negative: "var(--chart-negative)",
  neutral: "var(--chart-neutral)",
  warning: "var(--warning)",
  info: "var(--info)",
} as const

export type ChartTone = keyof typeof CHART_TONES

export function chartTone(tone: ChartTone): string {
  return CHART_TONES[tone]
}

/** The token an annotation's tone paints in. */
export function annotationPaint(tone: ChartAnnotationTone = "neutral"): string {
  return ANNOTATION_PAINT[tone] ?? ANNOTATION_PAINT.neutral
}

/** The numbers an annotation pins to the value axis, so the scale can hold them. */
export function annotationValues(annotation: ChartAnnotation): number[] {
  if (annotation.kind === "line" && annotation.axis !== "x" && typeof annotation.value === "number")
    return [annotation.value]
  return []
}

/**
 * Every annotation as a line of text, appended to a chart's visually hidden
 * rows: a goal line a sighted reader can see has to be readable too.
 */
export function annotationRows(
  annotations: ChartAnnotation[] = [],
  options: {
    valueFormatter?: (n: number) => string
    indexFormatter?: (v: string | number) => string
  } = {}
): string[] {
  const {
    valueFormatter = defaultValueFormatter,
    indexFormatter = defaultIndexFormatter,
  } = options
  const at = (value: number | string) =>
    typeof value === "number" ? valueFormatter(value) : indexFormatter(value)

  return annotations.map((annotation) => {
    if (annotation.kind === "line")
      return `${annotation.label}: ${
        annotation.axis === "x" ? indexFormatter(annotation.value) : at(annotation.value)
      }`
    if (annotation.kind === "event") return `${annotation.label} at ${indexFormatter(annotation.x)}`
    return `${annotation.label}: ${indexFormatter(annotation.from)} to ${indexFormatter(annotation.to)}`
  })
}

import {
  annotationValues,
  type ChartAnnotation,
  type ChartDatum,
  type ChartSeries,
} from "@/components/ui/chart-core"

// A printed scale lands on steps a reader can add up in their head. Anything
// else — recharts' own 0/650/1.3K/2K/2.6K — makes the reader do arithmetic to
// place a point between two gridlines.
const NICE_STEPS = [1, 2, 2.5, 5, 10] as const

/** Rounds a rough interval up to the next 1, 2, 2.5 or 5 × 10ⁿ. */
export function niceStep(rough: number): number {
  if (!Number.isFinite(rough) || rough <= 0) return 1
  const magnitude = 10 ** Math.floor(Math.log10(rough))
  const scaled = rough / magnitude
  const step = NICE_STEPS.find((candidate) => scaled <= candidate * (1 + 1e-9)) ?? 10
  return step * magnitude
}

// Summing a float step accumulates error, so each tick is computed from the
// index and then snapped back to the precision the step itself carries.
const snap = (value: number): number => Number(value.toPrecision(12))

/**
 * Where a value axis starts. `zero` — the default everywhere — always takes
 * zero in, because a count, a sum or a share is read against nothing, and a
 * bar that does not start there lies about its length. `auto` fits the axis
 * to the data's own range, on the same nice steps, for a level whose
 * movement is the reading — a rate, a price, a temperature — and which a
 * zero floor would flatten into a line along the top of the plot.
 */
export type ChartBaseline = "zero" | "auto"

/** Ticks from `bottom` to `top` on `step`, both ends included. */
function ticksBetween(bottom: number, top: number, step: number): number[] {
  const ticks: number[] = []
  for (let i = 0; bottom + i * step <= top + step / 2; i += 1) ticks.push(snap(bottom + i * step))
  return ticks
}

/** The ticks and the domain for a range, on nice steps, both ends included. */
export function niceDomain(
  min: number,
  max: number,
  count = 4,
  options: { baseline?: ChartBaseline } = {}
): { domain: [number, number]; ticks: number[] } {
  if (options.baseline === "auto") return fittedDomain(min, max, count)

  const low = Math.min(0, Number.isFinite(min) ? min : 0)
  const high = Math.max(Number.isFinite(max) ? max : 0, low)
  // A flat series at zero still needs two gridlines, or the plot has no scale.
  if (high === low) return { domain: [low, low + 1], ticks: [low, low + 1] }

  const step = niceStep((high - low) / Math.max(1, count))
  const bottom = Math.floor(low / step + 1e-9) * step
  const top = Math.ceil(high / step - 1e-9) * step
  return { domain: [snap(bottom), snap(top)], ticks: ticksBetween(bottom, top, step) }
}

/**
 * The `auto` baseline: the data's own range, widened out to the nice steps
 * either side of it, so 1.05–1.11 prints 1.04 / 1.06 / … / 1.12 rather than
 * 0 / 0.5 / 1 / 1.5. A flat level still gets a scale — a step either side of
 * it — and nothing to plot falls back to the zero baseline's 0 / 1.
 *
 * A peak drawn on the frame's edge reads as clipped, so an extreme within a
 * quarter of a step of the edge gets one step more: 1.05–1.12 prints up to
 * 1.14. That air never takes the axis across zero, which a level that does
 * not cross it has no business reaching past.
 */
function fittedDomain(min: number, max: number, count: number): { domain: [number, number]; ticks: number[] } {
  const lowest = Number.isFinite(min) ? min : 0
  const highest = Number.isFinite(max) ? Math.max(max, lowest) : lowest
  let low = lowest
  let high = highest
  if (high === low) {
    if (low === 0) return niceDomain(0, 0, count)
    const pad = niceStep(Math.abs(low) / 10)
    low -= pad
    high += pad
  }

  const step = niceStep((high - low) / Math.max(1, count))
  let bottom = Math.floor(low / step + 1e-9) * step
  let top = Math.ceil(high / step - 1e-9) * step
  if (lowest - bottom < step / 4) bottom = lowest >= 0 ? Math.max(0, bottom - step) : bottom - step
  if (top - highest < step / 4) top = highest <= 0 ? Math.min(0, top + step) : top + step
  return { domain: [snap(bottom), snap(top)], ticks: ticksBetween(bottom, top, step) }
}

/** The printed scale from 0 to `max`: `niceTicks(2_900, 4)` is 0, 1000, 2000, 3000. */
export function niceTicks(max: number, count = 4): number[] {
  return niceDomain(0, max, count).ticks
}

/**
 * The lowest and highest number a set of series reaches, stacked or laid over
 * one another. On the zero baseline the extent always takes zero in, so a
 * hole in the data or a series of nothing reads as 0 / 0; on `auto` it is the
 * data's own range, and ±Infinity when there is nothing to read.
 */
export function chartExtent(
  data: ChartDatum[],
  series: ChartSeries[],
  options: { stacked?: boolean; compare?: boolean; baseline?: ChartBaseline } = {}
): { min: number; max: number } {
  const keys = series.flatMap((entry) =>
    options.compare && entry.compareKey ? [entry.key, entry.compareKey] : [entry.key]
  )
  const fitted = options.baseline === "auto"
  let min = fitted ? Infinity : 0
  let max = fitted ? -Infinity : 0
  for (const datum of data) {
    let stack = 0
    // A row of holes is no stack at all — not a stack of zero, which a
    // fitted axis would otherwise have to reach down to.
    let stacked = false
    for (const key of keys) {
      const raw = datum[key]
      if (typeof raw !== "number" || !Number.isFinite(raw)) continue
      if (options.stacked) {
        stack += raw
        stacked = true
      } else {
        if (raw < min) min = raw
        if (raw > max) max = raw
      }
    }
    if (stacked) {
      if (stack < min) min = stack
      if (stack > max) max = stack
    }
  }
  return { min, max }
}

/** A y axis' explicit domain and ticks for the data it has to hold. */
export function yAxisScale(
  data: ChartDatum[],
  series: ChartSeries[],
  options: {
    stacked?: boolean
    compare?: boolean
    count?: number
    annotations?: ChartAnnotation[]
    /** Where the axis starts; zero unless the chart asks to fit its data. */
    baseline?: ChartBaseline
  } = {}
): { domain: [number, number]; ticks: number[] } {
  const { min, max } = chartExtent(data, series, options)
  // A goal line above every plotted point still has to fit inside the plot.
  let low = min
  let high = max
  for (const annotation of options.annotations ?? []) {
    for (const value of annotationValues(annotation)) {
      if (value < low) low = value
      if (value > high) high = value
    }
  }
  return niceDomain(low, high, options.count ?? 4, { baseline: options.baseline })
}

// Below these the axis band costs more than the scale it prints: the labels
// crowd the plot at 120px tall and eat a third of the width at 240px wide.
const MIN_AXIS_HEIGHT = 120
const MIN_AXIS_WIDTH = 240

/**
 * Whether a y axis fits. Width 0 means "not measured yet" — on the server, and
 * in a test with no layout — and is never taken as "too narrow".
 */
export function fitsYAxis(height: number, width = 0): boolean {
  if (height < MIN_AXIS_HEIGHT) return false
  return width === 0 || width >= MIN_AXIS_WIDTH
}