drawing points on a map with canvas

This is basically the same as my last block, except the circles are drawn using canvas instead of SVG. Helpful method for large datasets, to avoid performance issues. The data binding method here comes from this great article by Irene Ros.

<!DOCTYPE html>
<head>
<title>A D3 Map, points plotted with canvas instead of SVG</title>
<style>

body {
  margin: 0;
}

#container {
  position: relative;
  overflow: hidden;
	z-index: 0;
}

#map{
	width:100%;
	height:100%;
}

#points{
	width:100%;
	height:100%;
	position:relative;
	z-index:100;
}
	
.layer{
	position:absolute;
	z-index:-10;
}

.tile {
	pointer-events: none;
	position: absolute;
	width: 256px;
	height: 256px;
}

.info {
  position: absolute;
  bottom: 0px;
  left: 0px;
	padding: 20px;
	background: #000;
	color: #fff;
	width: 100%;
	height: 18px;
	z-index: 1000;
	font-family:Helvetica, Arial, sans-serif;
	font-size:16px;
}

.credit{
	position:absolute;
  bottom: 0px;
  right: 0px;
	padding: 9px 20px;
	color:#fff;
	font-family:Helvetica, Arial, sans-serif;
	font-size:13px;
	z-index: 1000;
}

.credit a{
	text-decoration:none;
	color:#ddd;
}
				
</style>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script src="https://d3js.org/d3.geo.tile.v0.min.js"></script>
</head>
<body>
<div id="container">
	<div class="layer"></div>
	<div id="map">
		<canvas>
			<div id="points"></div>
		</canvas>
	</div>
	<div class="credit"><p>Data &copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> Tiles &copy; <a href="https://cartodb.com/attributions">CartoDB</a></p></div>
</div>
  <script type='text/javascript'>

/* global variables and initial canvas space set up*/

width = Math.max(960, window.innerWidth),
height = Math.max(500, window.innerHeight),
prefix = prefixMatch(["webkit", "ms", "Moz", "O"]);

d3.csv("ftgreenetrees.csv", function(error, dataset) { createMap(dataset) });

var tile = d3.geo.tile()
    .size([width, height]);

var projection = d3.geo.mercator()
.scale((1 << 24) / 2 / Math.PI)
.translate([-width / 2, -height / 2]); // just temporary

var zoom = d3.behavior.zoom()
    .scale(projection.scale() * 2 * Math.PI)
    .scaleExtent([1 << 9, 1 << 25])
    .translate(projection([-73.975536, 40.691674]).map(function(x) { return -x; }))
    .on("zoom", zoomed);

var container = d3.select("#container")
    	.style("width", width + "px")
    	.style("height", height + "px")
			.call(zoom)
	    .on("mousemove", mousemoved);
				
var base = d3.select('#map');

var chart = d3.select('canvas')
			.attr("class", "layer")
      .attr('width', width)
      .attr('height', height);

var context = chart.node().getContext('2d');

var locations = d3.select('#points');

var layer = d3.select('.layer');

var info = base.append("div")
    .attr("class", "info");

zoomed();

function createMap(dataset) {
	
  var dataBinding = locations.selectAll("points.arc")
    .data(dataset)
  
  		.enter()
      .append("points")
      .classed("arc", true)
      .attr("x", function(d) {return projection([d.y,d.x])[0]})
      .attr("y", function(d) {return projection([d.y,d.x])[1]})
      .attr("radius", 8)
      .attr("fillStyle", "#ff0000")
  drawCanvas();
}

function drawCanvas() {

var elements = locations.selectAll("points.arc");
  	elements.each(function(d) {
    var node = d3.select(this);
		
    context.beginPath();
		context.arc(node.attr("x"), node.attr("y"), node.attr("radius"), 0, 2 * Math.PI);
		context.fillStyle = node.attr("fillStyle");
    context.fill();
    context.closePath();
	})
}

function reDraw() {
	context.clearRect(0, 0, width, height);
	drawCanvas();
}

function zoomed() {
  var tiles = tile
      .scale(zoom.scale())
      .translate(zoom.translate())
      ();

projection
      .scale(zoom.scale() / 2 / Math.PI)
      .translate(zoom.translate());

d3.selectAll("points.arc")
			.attr("x", function(d) {return projection([d.y,d.x])[0]})
      .attr("y", function(d) {return projection([d.y,d.x])[1]})
			reDraw();
			
  var image = layer
      .style(prefix + "transform", matrix3d(tiles.scale, tiles.translate))
    .selectAll(".tile")
      .data(tiles, function(d) { return d; });

image.exit()
      .remove();

image.enter().append("img")
      .attr("class", "tile")
      .attr("src", function(d) { return "https://" + ["a", "b", "c"][Math.random() * 3 | 0] + ".basemaps.cartocdn.com/light_all/" + d[2] + "/" + d[0] + "/" + d[1] + ".png"; })
      .style("left", function(d) { return (d[0] << 8) + "px"; })
      .style("top", function(d) { return (d[1] << 8) + "px"; });
}

function mousemoved() {
  info.text(formatLocation(projection.invert(d3.mouse(this)), zoom.scale()));
}

function matrix3d(scale, translate) {
  var k = scale / 256, r = scale % 1 ? Number : Math.round;
  return "matrix3d(" + [k, 0, 0, 0, 0, k, 0, 0, 0, 0, k, 0, r(translate[0] * scale), r(translate[1] * scale), 0, 1 ] + ")";
}

function prefixMatch(p) {
  var i = -1, n = p.length, s = document.body.style;
  while (++i < n) if (p[i] + "Transform" in s) return "-" + p[i].toLowerCase() + "-";
  return "";
}

function formatLocation(p, k) {
  var format = d3.format("." + Math.floor(Math.log(k) / 2 - 2) + "f");
  return (p[1] < 0 ? format(-p[1]) + " S" : format(p[1]) + " N") + " "
       + (p[0] < 0 ? format(-p[0]) + " W" : format(p[0]) + " E");
}

</script>
</body>
</html>

Modified http://d3js.org/d3.v3.min.js to a secure url

Modified http://d3js.org/d3.geo.tile.v0.min.js to a secure url

https://d3js.org/d3.v3.min.js

https://d3js.org/d3.geo.tile.v0.min.js