text: function pca_chart(pca){
      var data = Object.keys(pca).map(function(key) {
          return {
              "component": pca[key]['Component'],
              "variance": pca[key]['Explained variance ratio']
          };
      });
  
  
      var margin = {top: 20, right: 20, bottom: 30, left: 40},
          width = 960 - margin.left - margin.right,
          height = 200 - margin.top - margin.bottom;
  
      var x = d3.scale.ordinal()
          .rangeRoundBands([0, width], .1);
  
      var y = d3.scale.linear()
          .range([height, 0]);
  
      var xAxis = d3.svg.axis()
          .scale(x)
          .orient("bottom");
  
      var yAxis = d3.svg.axis()
          .scale(y)
          .orient("left")
          .ticks(10);
  
      var svg = d3.select("div.pca_chart").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 + ")");
  
  
      x.domain(data.map(function(d) { return d.component; }));
      y.domain([0, d3.max(data, function(d) { return d.variance; })]);
  
      svg.append("g")
        .attr("class", "x axis")
        .attr("transform", "translate(0," + height + ")")
        .call(xAxis)
      .append("text")
        .attr("y", 15)
        .style("text-anchor", "middle")
        .text("Component");
  
      svg.append("g")
        .attr("class", "y axis")
        .call(yAxis)
      .append("text")
        .attr("transform", "rotate(-90)")
        .attr("y", 6)
        .attr("dy", ".71em")
        .style("text-anchor", "end")
        .text("Explained variance ratio");
  
      svg.selectAll(".bar")
        .data(data)
      .enter().append("rect")
        .attr("class", "bar")
        .attr("x", function(d) { return x(d.component); })
        .attr("width", x.rangeBand())
        .attr("y", function(d) { return y(d.variance); })
        .attr("height", function(d) { return height - y(d.variance); });
  
  }
  