function main() {
  // This is a reimplementation of the Grouped Bar Chart by Mike Bostock
  // (http://bl.ocks.org/882152). Although useful, I found the original's
  // minimal comments and inverted axes hard to follow, so I created the
  // version you see here.

  // First, we define sizes and colours...
  var outerW = 640; // outer width
  var outerH = 480; // outer height
  var padding = { t: 0, r: 0, b: 0, l: 0 };
  var w = outerW - padding.l - padding.r; // inner width
  var h = outerH - padding.t - padding.b; // inner height
  var c = [ "#E41A1C", "#377EB8", "#4DAF4A" ]; // ColorBrewer Set 1

  // Second, we define our data...
  // Create a two-dimensional array.
  // The first dimension has as many Array elements as there are series.
  // The second dimension has as many Number elements as there are groups.
  // It looks something like this...
  //  var data = [
  //    [ 0.10, 0.09, 0.08, 0.07, 0.06, ... ], // series 1
  //    [ 0.10, 0.09, 0.08, 0.07, 0.06, ... ], // series 2
  //    [ 0.10, 0.09, 0.08, 0.07, 0.06, ... ]  // series 3
  //  ];
  var numberGroups = 10; // groups
  var numberSeries = 3;  // series in each group
  var data = d3.range(numberSeries).map(function () { return d3.range(numberGroups).map(Math.random); });

  // Third, we define our scales...
  // Groups scale, x axis
  var x0 = d3.scale.ordinal()
      .domain(d3.range(numberGroups))
      .rangeBands([0, w], 0.2);

  // Series scale, x axis
  // It might help to think of the series scale as a child of the groups scale
  var x1 = d3.scale.ordinal()
      .domain(d3.range(numberSeries))
      .rangeBands([0, x0.rangeBand()]);

  // Values scale, y axis
  var y = d3.scale.linear()
      .domain([0, 1]) // Because Math.random returns numbers between 0 and 1
      .range([0, h]);

  // Visualisation selection
  var vis = d3.select("#vis")
      .append("svg:svg")
      .attr("width", outerW)
      .attr("height", outerH);

  // Series selection
  // We place each series into its own SVG group element. In other words,
  // each SVG group element contains one series (i.e. bars of the same colour).
  // It might be helpful to think of each SVG group element as containing one bar chart.
  var series = vis.selectAll("g.series")
      .data(data)
    .enter().append("svg:g")
      .attr("class", "series") // Not strictly necessary, but helpful when inspecting the DOM
      .attr("fill", function (d, i) { return c[i]; })
      .attr("transform", function (d, i) { return "translate(" + x1(i) + ")"; });

  // Groups selection
  var groups = series.selectAll("rect")
      .data(Object) // The second dimension in the two-dimensional data array
    .enter().append("svg:rect")
        .attr("x", 0)
        .attr("y", function (d) { return h - y(d); })
        .attr("width", x1.rangeBand())
        .attr("height", y)
        .attr("transform", function (d, i) { return "translate(" + x0(i) + ")"; });
}