Pulsing circles with d3 bro

circles.js

var pi = Math.PI,
    sin = Math.sin,
    cos = Math.cos

var t = 0;

setInterval(function(){
    var data = [sin(t)+1, cos(t)+1, sin(t)+1, cos(t)+1, sin(t)+1];
    
    // bind circle elements to data
    var circles = d3.selectAll(".chart")
        .selectAll("circle")
        .data(data);
        
    // create circles if there aren't any
    circles.enter().append("circle")
        .attr("cx", function(d, i){
            return 100 + i*100;
        })
        .attr("cy", 100);

    // change the radius of each circle based on da data
    circles.attr("r", function(d){
        return d*10 + 10;
    });
    
    t += 0.1;
    if(t > 2*pi){
        t = 0;
    }
}, 10);

index.html

<style>
circle {
    fill: black;
}
</style>

<script src="http://d3js.org/d3.v3.min.js" charset="utf-8"></script>
<script src="circles.js"></script>

<svg class="chart" width="720" height="200"></svg>