var s = d3.select('#vis') //selection

var t = 0 // increment
var w = 960
var h = 400
var reps = 1500 // number of repetitions 
var colorRainbow = d3.scaleSequential()
    .domain([0, reps])
    .interpolator(d3.interpolateRainbow)

function update(_t) {
    s.append('circle')
        .attrs({
            cx: x(_t),
            cy: y(_t) + (h/2),
            r: d3.randomUniform(2, 7)(),
            fill: colorRainbow(_t)
        })
}

// parametric equations
function x(n) {
    return Math.sin(n / 10) * 100 + Math.sin(n / 15) * 100
}

function y(n) {
    return Math.cos(n / 10) * 100
}

// update
d3.interval(function() {
    if (t < reps) {
        t++
        update(t)
    }
}, d3.randomUniform(8, 24)());