  height = 400 - margin.top - margin.bottom;

// append the svg object to the body of the page
var svg = d3.select("#my_dataviz")
  .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 + ")");

// Initialize the X axis
var x = d3.scaleBand()
  .range([ 0, width ])
  .padding(0.2);
var xAxis = svg.append("g")
  .attr("transform", "translate(0,