Mark a location on an svg map in d3.js - svg

Question

I am using this example from the d3.js wiki.
http://bl.ocks.org/2206590
From that, I have a map, and I want to know how to mark a single location on it.
How do I plot a small circle on this map at the location with co-ordinates [40.717079,-74.009628].
Here is the source code from the example:
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
svg.append("rect")
.attr("class", "background")
.attr("width", width)
.attr("height", height)
.on("click", click);
var g = svg.append("g")
.attr("transform", "translate(" + width / 2 + "," + height / 2 + ")")
.append("g")
.attr("id", "states");
var projection = d3.geo.albersUsa()
.scale(width)
.translate([0, 0]);
var path = d3.geo.path()
.projection(projection);
d3.json("readme.json", function(json) {
g.selectAll("path")
.data(json.features)
.enter().append("path")
.attr("d", path);
});

Answer 1

OK this may not be the right answer, but this is what I did:
var width = 1060,
height = 600,
centered;
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
svg.append("rect")
.attr("class", "background")
.attr("width", width)
.attr("height", height)
.on("click", click);
var g = svg.append("g")
.attr("transform", "translate(" + width / 2 + "," + height / 2 + ")")
.append("g")
.attr("id", "states");
var projection = d3.geo.albersUsa()
.scale(width)
.translate([0, 100]);
var path = d3.geo.path()
.projection(projection);
setInterval(function(){
draw();
},1000);
function draw(){
d3.json("readme.json", function(json) {
g.selectAll("path")
.data(json.features)
.enter()
.append("path")
.attr("d", path)
.on("click", click);
var latitude = 35 + Math.floor(Math.random()*8);
var longitude = -1 * (83 + Math.floor(Math.random()*35));
var coordinates = projection([longitude, latitude]);
g.append('svg:circle')
.attr('cx', coordinates[0])
.attr('cy', coordinates[1])
.attr('r', 2)
.attr('stroke','red')
.attr('fill','red');
});
}
The code works, but I suspect I am doing the wrong thing by redrawing the whole map every time I render a new co-ordinate.