D3 Examples - Hexbin map of Boston crime

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<!DOCTYPE html>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
    <meta name="viewport" content="width=1024, user-scalable=no">
    <style>
      html { height: 100% }
      body { height: 100%; margin: 0; padding: 0;}
      #map{ height: 100% }
    </style>
<link rel="stylesheet" href="https://cdn.leafletjs.com/leaflet-0.5.1/leaflet.css" />
<title>Hex Bins + leaflet</title>
 
<div id="map"></div>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script src="colorbrewer.js"></script>
<script src="hexbin.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/leaflet/0.5.1/leaflet.min.js"></script>
<script>
var m = L.map("map").setView([42.35321607653588, -71.06369018554688],11);
var buckets = 11;
//make the map
var _p;
L.layerGroup([
  L.tileLayer("https://{s}.tile.stamen.com/terrain-background/{z}/{x}/{y}.jpg",{minZoom:4,maxZoom:18}),
  L.tileLayer("https://{s}.tile.stamen.com/terrain-lines/{z}/{x}/{y}.png",{minZoom:4,maxZoom:18,attribution:'Map tiles by <a href="https://stamen.com">Stamen Design</a>, <a href="https://creativecommons.org/licenses/by/3.0">CC BY 3.0</a> &mdash; Map data &copy; <a href="https://openstreetmap.org">OpenStreetMap</a> contributors, <a href="https://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>'})
    ]).addTo(m);
    
//add a tileset 
//boston police bloter
d3.json("points.json",function(err,points){
    _p = points;
//generate 60k random points
var radius = 0.0035;
var hexbin = d3.hexbin()
    .size([
        d3.max(points,function(v){return v[0]})-d3.min(points,function(v){return v[0]}), 
        d3.max(points,function(v){return v[1]})-d3.min(points,function(v){return v[1]})])
    .radius(radius);
​
//create a hexbin
​
var hex = hexbin(points);
​
//populate it
​
var q = d3.scale.quantile().domain(hex.map(function(h){return h.length;}).reduce(function(a,b){if(a.indexOf(b)===-1){a.push(b)};return a;},[]).sort(function(a,b){return a-b})).range(d3.range(buckets))
​
//make a scale
​
function hexagon(radius) {
    var x0 = 0, y0 = 0;
    return d3.range(0, 2 * Math.PI, Math.PI / 3).map(function(angle) {
      var x1 = Math.sin(angle) * radius,
          y1 = -Math.cos(angle) * radius,
          dx = x1 - x0,
          dy = y1 - y0;
      x0 = x1, y0 = y1;
      return [dx, dy];
    });
};
​
//from the hexbin file, we need this to create points
​
var transform = hexagon(radius);
​
//use it to create a transform function
 transform.shift();
 
//remove the first value as it's a duplicate
​
L.layerGroup(hex.map(function(h){//create a layergroup
    return L.polygon(transform.map(function(t){//filled with polygons
        return [h.x+t[0],h.y+t[1]];//that are the hexagons we need
    }),{//styled like so
        stroke:false,
        color:colorbrewer.Spectral[buckets][(buckets-1)-q(h.length)],//including color brewer colors
        fillOpacity:0.7
    }).bindPopup("number: "+h.length);//with the number of points in the popup
})).addTo(m);
});
</script>
    