Geo map

A map is the right chart only when the geography is the question. Population usually explains the pattern, so a choropleth of raw counts is a map of where people live redrawn with your metric's name on it.


Tokens this needs: --foreground, --card, --muted, --border, --chart-1, --scale-seq-3
The status, chart, scale, state and direction names are an extension,
not a rename. shadcn has --destructive and five --chart-* and nothing
else in this territory.

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// GeoMap.tsx
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/**
 * Markers for discrete located things. They fail by overlapping, so past a
 * pixel radius they cluster, and a cluster carries its count and scales with
 * it: five and five hundred have to look different. The base map is one
 * outline in a muted stroke, because it is context and should recede.
 *
 * Plain equirectangular projection. Good enough for a country; wrong for a
 * globe, where you want a real projection library.
 */
export type Point = { id: string; lat: number; lng: number; label: string };
export type Bounds = { lat: [number, number]; lng: [number, number] };
export type Cluster = { x: number; y: number; points: Point[] };

export function cluster(points: { x: number; y: number; point: Point }[], radius: number): Cluster[] {
  const out: Cluster[] = [];
  for (const p of points) {
    const near = out.find((c) => Math.hypot(c.x - p.x, c.y - p.y) <= radius);
    if (!near) { out.push({ x: p.x, y: p.y, points: [p.point] }); continue; }
    const n = near.points.length;
    near.x = (near.x * n + p.x) / (n + 1);
    near.y = (near.y * n + p.y) / (n + 1);
    near.points.push(p.point);
  }
  return out;
}

export function GeoMap({ points, bounds, outline, width = 320, height = 240, clusterRadius = 24, onSelect }: {
  points: Point[];
  /** The viewport, as a latitude range and a longitude range. */
  bounds: Bounds;
  /** Coastline or region border as lat/lng vertices, drawn under the data. */
  outline?: [number, number][];
  width?: number;
  height?: number;
  /** Pixel distance under which markers merge. */
  clusterRadius?: number;
  /** A marker gives one point; a cluster gives all of them. */
  onSelect?: (points: Point[]) => void;
}) {
  const [lat0, lat1] = bounds.lat, [lng0, lng1] = bounds.lng;
  const squeeze = Math.cos(((lat0 + lat1) / 2) * Math.PI / 180);
  const k = Math.min(width / ((lng1 - lng0) * squeeze), height / (lat1 - lat0));
  const ox = (width - (lng1 - lng0) * squeeze * k) / 2, oy = (height - (lat1 - lat0) * k) / 2;
  const project = (lat: number, lng: number) => ({ x: ox + (lng - lng0) * squeeze * k, y: oy + (lat1 - lat) * k });

  const clusters = cluster(points.map((p) => ({ ...project(p.lat, p.lng), point: p })), clusterRadius);
  const r = (n: number) => (n === 1 ? 4 : 4 + 1.6 * Math.sqrt(n));

  return (
    <svg width={width} height={height} viewBox={`0 0 ${width} ${height}`} role="img" aria-label={`${points.length} locations on a map`} className="block max-w-full">
      {outline && (
        <path
          d={outline.map(([lat, lng], i) => { const p = project(lat, lng); return `${i ? "L" : "M"}${p.x.toFixed(1)} ${p.y.toFixed(1)}`; }).join(" ") + " Z"}
          className="fill-muted/40 stroke-border"
          strokeWidth={1}
        />
      )}
      {clusters.map((c) => {
        const n = c.points.length;
        return (
          <g key={c.points[0].id} onClick={onSelect && (() => onSelect(c.points))} className={onSelect && "cursor-pointer"}>
            <circle cx={c.x.toFixed(1)} cy={c.y.toFixed(1)} r={r(n).toFixed(1)} className={n === 1 ? "fill-chart-1" : "fill-scale-seq-3"}>
              <title>{n === 1 ? c.points[0].label : `${n} locations`}</title>
            </circle>
            {n > 1 && (
              <text x={c.x.toFixed(1)} y={(c.y + 3).toFixed(1)} textAnchor="middle" className="fill-foreground text-[9px] font-medium tabular-nums">{n}</text>
            )}
          </g>
        );
      })}
    </svg>
  );
}

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// stores.ts
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import type { Point } from "./GeoMap";

/** Forty-six stores: forty-two inside Greater London and four elsewhere. */
export const STORES: Point[] = [
  ["oxford-st", 51.515, -0.141, "Oxford Street"], ["covent-garden", 51.512, -0.123, "Covent Garden"],
  ["stratford", 51.543, -0.008, "Stratford"], ["white-city", 51.507, -0.221, "White City"],
  ["canary-wharf", 51.505, -0.019, "Canary Wharf"], ["kings-cross", 51.531, -0.124, "King's Cross"],
  ["camden", 51.539, -0.143, "Camden"], ["angel", 51.532, -0.106, "Angel"],
  ["hackney", 51.545, -0.055, "Hackney"], ["brixton", 51.462, -0.115, "Brixton"],
  ["clapham", 51.462, -0.138, "Clapham"], ["wimbledon", 51.421, -0.206, "Wimbledon"],
  ["richmond", 51.461, -0.304, "Richmond"], ["kingston", 51.41, -0.306, "Kingston"],
  ["croydon", 51.372, -0.099, "Croydon"], ["bromley", 51.406, 0.014, "Bromley"],
  ["greenwich", 51.478, 0.0, "Greenwich"], ["lewisham", 51.465, -0.013, "Lewisham"],
  ["peckham", 51.474, -0.069, "Peckham"], ["ealing", 51.513, -0.305, "Ealing"],
  ["hammersmith", 51.492, -0.223, "Hammersmith"], ["kensington", 51.501, -0.192, "Kensington"],
  ["chelsea", 51.487, -0.169, "Chelsea"], ["victoria", 51.496, -0.144, "Victoria"],
  ["waterloo", 51.503, -0.113, "Waterloo"], ["london-bridge", 51.505, -0.086, "London Bridge"],
  ["liverpool-st", 51.518, -0.081, "Liverpool Street"], ["shoreditch", 51.526, -0.078, "Shoreditch"],
  ["bethnal-green", 51.527, -0.055, "Bethnal Green"], ["walthamstow", 51.583, -0.02, "Walthamstow"],
  ["wood-green", 51.597, -0.11, "Wood Green"], ["finchley", 51.599, -0.187, "Finchley"],
  ["hampstead", 51.556, -0.178, "Hampstead"], ["harrow", 51.579, -0.335, "Harrow"],
  ["uxbridge", 51.546, -0.478, "Uxbridge"], ["hounslow", 51.468, -0.361, "Hounslow"],
  ["sutton", 51.361, -0.194, "Sutton"], ["ilford", 51.559, 0.07, "Ilford"],
  ["romford", 51.575, 0.183, "Romford"], ["barking", 51.54, 0.081, "Barking"],
  ["woolwich", 51.491, 0.069, "Woolwich"], ["enfield", 51.652, -0.081, "Enfield"],
  ["manchester", 53.48, -2.24, "Manchester"], ["birmingham", 52.48, -1.9, "Birmingham"],
  ["bristol", 51.45, -2.59, "Bristol"], ["edinburgh", 55.95, -3.19, "Edinburgh"],
].map(([id, lat, lng, label]) => ({ id, lat, lng, label } as Point));

/** A coarse Great Britain coastline, enough to place the markers and no more. */
export const GREAT_BRITAIN: [number, number][] = [
  [50.07, -5.7], [50.37, -4.14], [50.52, -2.45], [50.82, -0.14], [51.13, 1.32], [51.38, 1.44],
  [51.96, 1.35], [52.48, 1.75], [52.93, 1.3], [52.9, 0.2], [53.15, 0.34], [53.58, 0.11],
  [54.12, -0.08], [54.49, -0.61], [54.69, -1.21], [55.0, -1.4], [55.77, -2.0], [56.0, -2.5],
  [56.05, -3.3], [56.4, -2.8], [57.15, -2.1], [57.7, -2.0], [57.5, -4.2], [58.44, -3.1],
  [58.62, -5.0], [57.9, -5.2], [57.3, -6.3], [56.8, -5.1], [55.3, -5.8], [55.46, -4.63],
  [54.9, -3.6], [54.5, -3.6], [54.1, -3.2], [53.8, -3.05], [53.4, -3.0], [53.4, -4.6],
  [52.8, -4.7], [52.4, -4.1], [51.9, -5.3], [51.6, -4.0], [51.45, -3.2], [51.35, -3.0],
  [51.2, -3.5], [51.0, -4.5], [50.55, -4.95],
];

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// demo.tsx
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import { GeoMap } from "./GeoMap";
import { GREAT_BRITAIN, STORES } from "./stores";

/**
 * Forty-six stores on a map of Great Britain. At this zoom the London stores
 * sit within a few pixels of each other, so they merge into one cluster that
 * carries the count; the four elsewhere stay as single markers.
 */
export default function Demo() {
  return (
    <div className="w-fit rounded-lg border bg-card p-2">
      <GeoMap
        points={STORES}
        bounds={{ lat: [49.9, 58.8], lng: [-6.5, 2.0] }}
        outline={GREAT_BRITAIN}
        width={320}
        height={240}
        onSelect={(points) => console.log(points.map((p) => p.label))}
      />
    </div>
  );
}