function ready(geo, userData) {
  // Some set up.
  const width = 900;
  const height = 500;
  const extentUk = [[0, height * 0.05], [width, height * 0.95]];
  const pr = window.devicePixelRatio || 1;

  // Crisp canvas and context.
  const canvas = d3.select('canvas')
    .attr('width', width * pr)
    .attr('height', height * pr)
    .style('width', `${width}px`);
  const context = canvas.node().getContext('2d');
  context.scale(pr, pr);

  // Background.
  context.fillStyle = '#02154e';
  context.fillRect(0, 0, width, height);

  // Projection and path.
  const projection = d3.geoConicEqualArea()
    .parallels([50, 61])
    .fitExtent(extentUk, geo);

  const geoPath = d3.geoPath()
    .projection(projection)
    .context(context);

  // Prep user data.
  userData.forEach(site => {
    const coords = projection([+site.lng, +site.lat]);
    site.x = coords[0];
    site.y = coords[1];
  });

  // Remove non UK mainland sites.
  const poly = d3.geoPolygon(geo, projection);
  userData = d3.polygonPoints(userData, poly);

  // Hexgrid generator.
  const hexgrid = d3.hexgrid()
    .extent(extentUk)
    .geography(geo)
    .projection(projection)
    .pathGenerator(geoPath)
    .hexRadius(3);

  // Hexgrid instanace.
  const hex = hexgrid(userData);

  // Colour scale.
  const counts = hex.grid.layout
    .map(el => el.datapointsWt)
    .filter(el => el > 0);
  const ckBreaks = ss.ckmeans(counts, 4).map(clusters => clusters[0]);
  const colour = d3.scaleThreshold()
    .domain(ckBreaks)
    .range(['#fff9f9', '#fbb1b0', '#fc8584', '#fd5958', '#fe2c2c', 'crimson']);

  // Draw prep.
  const hexagon = new Path2D(hex.hexagon());

  // Draw.
  hex.grid.layout.forEach(hex => {
    context.save();
    context.translate(hex.x, hex.y);
    context.fillStyle = colour(hex.datapointsWt);
    context.fill(hexagon);
    context.restore();
  });
}

// Load data.
const uk = d3.json(
  'https://raw.githubusercontent.com/larsvers/map-store/master/GBR_adm0_topo.json'
);
const points = d3.csv(
  'https://raw.githubusercontent.com/larsvers/data-store/master/postbox_locations_uk.csv'
);

Promise.all([uk, points]).then(res => {
  let [geoData, userData] = res;

  // Turn Topo to GeoJSON.
  const ukGeo = topojson.feature(geoData, geoData.objects.GBR_adm0);

  ready(ukGeo, userData);
});