density.js

// Create the map
var map = new google.maps.Map(d3.select("#map").node(), {
	zoom : 11,
	// center: new google.maps.LatLng(37.3943454817, -122.078649925),
	center : new google.maps.LatLng(-33.778291, 151.168184),
	mapTypeId : google.maps.MapTypeId.ROADMAP
});

// Load the location data. When the data comes back, create an overlay.
d3.json("destinations2.json", function(data) {

	// Simplistic 95% calculation for max
	var sdata = _.sortBy(data, function(d) {
		return d.count;
	});
	var i = Math.round(0.95 * sdata.length);
	var max = sdata[i].count;

	var color = d3.scale.log().domain([ 1, max ]).range(
					[ "rgb(255, 255, 229)", "rgb(35, 139, 69)" ]);

	// Draw a rectangle for every data record
	_.each(data, function(d) {
		var rectangle = new google.maps.Rectangle();
		var center = new google.maps.LatLng(d.center[0], d.center[1]);
		var nw = google.maps.geometry.spherical.computeOffset(center, 500 * Math.sqrt(2), 315);
		var se = google.maps.geometry.spherical.computeOffset(center, 500 * Math.sqrt(2), 135);
		var bounds = new google.maps.LatLngBounds();
		bounds.extend(se);
		bounds.extend(nw);
		var rectOptions = {
			strokeColor : "#AAAAAA",
			//strokeColor : "blue",
			strokeOpacity : 0.8,
			strokeWeight : 2,
			fillColor : color(d.count),
			fillOpacity : 0.35,
			fillOpacity : 0.60,
			map : map,
			bounds : bounds
		};
		rectangle.setOptions(rectOptions);
	})
});

destinations2.json

[{"x":12749.0,"y":-4209.0,"count":2.0,"center":[-37.8100903085907,144.961257547313]}, 
{"x":13547.0,"y":-3930.0,"count":5.0,"center":[-35.3037906658972,149.117592639439]}, 
{"x":13551.0,"y":-3928.0,"count":1.0,"center":[-35.2858243602148,149.128515399598]}, 
{"x":13553.0,"y":-3927.0,"count":5.0,"center":[-35.2768412073736,149.133980459762]}, 
{"x":13961.0,"y":-3778.0,"count":3.0,"center":[-33.9383514340355,151.166750778159]}, 
{"x":13963.0,"y":-3755.0,"count":124.0,"center":[-33.731738918688,150.823387255003]}, 
{"x":13968.0,"y":-3756.0,"count":3.0,"center":[-33.7407220715292,150.893194126261]}, 
{"x":13971.0,"y":-3773.0,"count":11.0,"center":[-33.8934356698295,151.195313588677]}, 
{"x":13972.0,"y":-3772.0,"count":1.0,"center":[-33.8844525169883,151.190212743657]}, 
{"x":13973.0,"y":-3772.0,"count":2.0,"center":[-33.8844525169883,151.201033686453]}, 
{"x":13974.0,"y":-3771.0,"count":2.0,"center":[-33.8754693641472,151.195936492494]}, 
{"x":13976.0,"y":-3770.0,"count":9.0,"center":[-33.866486211306,151.20166275278]}, 
{"x":13976.0,"y":-3771.0,"count":3.0,"center":[-33.8754693641472,151.217576099835]}, 
{"x":13977.0,"y":-3771.0,"count":2.0,"center":[-33.8754693641472,151.228395903505]}, 
{"x":13978.0,"y":-3769.0,"count":1.0,"center":[-33.8575030584648,151.207391524537]}, 
{"x":13979.0,"y":-3772.0,"count":186.0,"center":[-33.8844525169883,151.265959343228]}, 
{"x":13980.0,"y":-3767.0,"count":3.0,"center":[-33.8395367527824,151.197226098097]}, 
{"x":13981.0,"y":-3767.0,"count":4.0,"center":[-33.8395367527824,151.208041350322]}, 
{"x":13982.0,"y":-3767.0,"count":4.0,"center":[-33.8395367527824,151.218856602546]}, 
{"x":13986.0,"y":-3760.0,"count":7.0,"center":[-33.776654682894,151.150990423843]}, 
{"x":13986.0,"y":-3761.0,"count":1.0,"center":[-33.7856378357352,151.166844592526]}, 
{"x":13986.0,"y":-3762.0,"count":3.0,"center":[-33.7946209885764,151.182705804154]}, 
{"x":13987.0,"y":-3760.0,"count":3.0,"center":[-33.776654682894,151.161797730465]}, 
{"x":13987.0,"y":-3761.0,"count":3.0,"center":[-33.7856378357352,151.177653032722]}, 
{"x":13987.0,"y":-3762.0,"count":2.0,"center":[-33.7946209885764,151.193515378428]}, 
{"x":13988.0,"y":-3760.0,"count":2.0,"center":[-33.776654682894,151.172605037088]}, 
{"x":13989.0,"y":-3760.0,"count":1.0,"center":[-33.776654682894,151.18341234371]}, 
{"x":13990.0,"y":-3758.0,"count":3.0,"center":[-33.7586883772116,151.16252336727]}, 
{"x":13997.0,"y":-3761.0,"count":9.0,"center":[-33.7856378357352,151.28573743469]}, 
{"x":13998.0,"y":-3752.0,"count":2.0,"center":[-33.7047894601644,151.153989335012]}, 
{"x":13999.0,"y":-3752.0,"count":5.0,"center":[-33.7047894601644,151.164787591144]}, 
{"x":14004.0,"y":-3752.0,"count":4.0,"center":[-33.7047894601644,151.218778871803]}] 

index.html

<!DOCTYPE html>
<html>
  <head>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no"/>
    <script type="text/javascript" src="http://maps.google.com/maps/api/js?libraries=geometry&sensor=true"></script>
    <script type="text/javascript" src="http://mbostock.github.com/d3/d3.js?1.29.1"></script>
    <script type="text/javascript" src="http://documentcloud.github.com/underscore/underscore-min.js"></script>

    <style type="text/css">

html, body, #map {
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
}

    </style>
  </head>
  <body>
    <div id="map"></div>
    <script type="text/javascript" src="density.js"></script>
  </body>
</html>