the X axis
  var groups = d3.map(data, function(d){return(d.group)}).keys()

  // Add X axis
  var x = d3.scaleBand()
      .domain(groups)
      .range([0, width])
      .padding([0.2])
  svg.append("g")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x).tickSizeOuter(0));

  // Add Y axis
  var y = d3.scaleLinear()
    .domain([0, 100])
    .range([ height, 0 ]);
  svg.append("g")
    .call(d3.axisLeft(y));

  // color palette = one color per subgroup
  var color = d3.scaleOr