graph.js

var n = 4, // number of layers
    m = 58, // number of samples per layer
    stack = d3.layout.stack(),
    layers = stack(d3.range(n).map(function() { return bumpLayer(m, .1); })),
    yGroupMax = d3.max(layers, function(layer) { return d3.max(layer, function(d) { return d.y; }); }),
    yStackMax = d3.max(layers, function(layer) { return d3.max(layer, function(d) { return d.y0 + d.y; }); });

var margin = {top: 40, right: 10, bottom: 20, left: 10},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.scale.ordinal()
    .domain(d3.range(m))
    .rangeRoundBands([0, width], .08);

var y = d3.scale.linear()
    .domain([0, yStackMax])
    .range([height, 0]);

var color = d3.scale.linear()
    .domain([0, n - 1])
    .range(["#aad", "#556"]);

var xAxis = d3.svg.axis()
    .scale(x)
    .tickSize(0)
    .tickPadding(6)
    .orient("bottom");

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
    .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("g")
    .attr("class", "x axis top")
    .attr("transform", "translate(0,"+height+")")
    .call(xAxis);

var layer = svg.selectAll(".layer")
    .data(layers)
    .enter().append("g")
    .attr("class", "layer")
    .style("fill", function(d, i) { return color(i); });

var rect = layer.selectAll("rect")
    .data(function(d) { return d; })
    .enter().append("rect")
    .attr("x", function(d) { return x(d.x); })
    .attr("y", height)
    .attr("width", x.rangeBand())
    .attr("height", 0);

rect.transition()
    .delay(function(d, i) { return i * 10; })
    .attr("y", function(d) { return y(d.y0 + d.y); })
    .attr("height", function(d) { return y(d.y0) - y(d.y0 + d.y); });

//svg.append("g")
//    .attr("class", "x axis bottom")
//    .attr("transform", "translate(0," + height + ")")
//    .call(xAxis);

d3.selectAll("input").on("change", change);

var timeout = setTimeout(function() {
    d3.select("input[value=\"grouped\"]").property("checked", true).each(change);
}, 2000);

function change() {
    clearTimeout(timeout);
    if (this.value === "grouped") transitionGrouped();
    else transitionStacked();
}

function transitionGrouped() {
    y.domain([0, yGroupMax]);

    xAxis.orient("top");
    svg.select('.x.axis').transition()
        .attr("transform", "translate(0,0)")
        .call(xAxis)
        .each('end', function(){
    rect.transition()
        .duration(500)
        .delay(function(d, i) { return i * 10; })
        .attr("x", function(d, i, j) { return x(d.x) + x.rangeBand() / n  * j; })
        //.attr('y', function(d) { return y(8)})
        .attr("width", x.rangeBand() / n)
        .transition()
        .attr("y", function(d) {
            return y(yGroupMax);
            //return y(d.y);
        })
        .attr("height", function(d) {
            return height-y(d.y);
            //return (height - y(d.y));
        });
    });
}

function transitionStacked() {
    y.domain([0, yStackMax]);
    xAxis.orient("bottom");
    svg.select('.x.axis').transition()
        .attr("transform", "translate(0,"+height+")")
        .call(xAxis).each('end', function(){
    rect.transition()
        .duration(500)
        .delay(function(d, i) { return i * 10; })
        .attr("y", function(d) { return y(d.y0 + d.y); })
        .attr("height", function(d) { return y(d.y0) - y(d.y0 + d.y); })
        .transition()
        .attr("x", function(d) { return x(d.x); })
        .attr("width", x.rangeBand());
    });
}

// Inspired by Lee Byron's test data generator.
function bumpLayer(n, o) {

    function bump(a) {
        var x = 1 / (.1 + Math.random()),
            y = 2 * Math.random() - .5,
            z = 10 / (.1 + Math.random());
        for (var i = 0; i < n; i++) {
            var w = (i / n - y) * z;
            a[i] += x * Math.exp(-w * w);
        }
    }

    var a = [], i;
    for (i = 0; i < n; ++i) a[i] = o + o * Math.random();
    for (i = 0; i < 5; ++i) bump(a);
    return a.map(function(d, i) { return {x: i, y: Math.max(0, d)}; });
}

index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>D3 Graph example</title>
    <style>
        body {
            font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
            margin: auto;
            position: relative;
            width: 960px;
        }

        text {
            font: 10px sans-serif;
        }

        .axis path,
        .axis line {
            fill: none;
            stroke: #000;
            shape-rendering: crispEdges;
        }

        form {
            position: absolute;
            right: 10px;
            top: 10px;
        }
    </style>
    <script src="//cdnjs.cloudflare.com/ajax/libs/d3/3.5.16/d3.min.js" type="application/javascript"></script>
</head>
<body>
<form>
    <label><input type="radio" name="mode" value="grouped"> Grouped</label>
    <label><input type="radio" name="mode" value="stacked" checked> Stacked</label>
</form>
<br/>
<script src="graph.js" type="application/javascript"></script>

</body>
</html>