flotr2 pie chart with broken down slices

HTML

<script src="https://rawgithub.com/HumbleSoftware/Flotr2/master/flotr2.min.js"></script>
<div id="example"></div>

CSS

#example {
  width: 400px;
  height: 300px;
}

JavaScript

var bigData = [{
  data: [
    [0, 6]
  ],
  label: 'a',
  sub: [{ // sub pie data
    data: [
      [0, 2]
    ],
    label: 'a1'
  }, {
    data: [
      [0, 4]
    ],
    label: 'a2'
  }]
}, {
  data: [
    [0, 4]
  ],
  label: 'b',
  sub: [{ // sub pie data
    data: [
      [0, 3]
    ],
    label: 'b1'
  }, {
    data: [
      [0, 1]
    ],
    label: 'b2'
  }]
}];

(function basic(container, data) {
  var graph;

  // Draw Graph
  graph = Flotr.draw(container, data, {
    title: 'Click a slice to see the details',
    pie: {
      show: true
    },
    mouse: {
      track: true,
      trackFormatter: function(obj) {
        return obj.series.label + ': ' + obj.y;
      }
    }
  });

  graph.observe(container, 'flotr:click', function(position) {
    var obj = position.hit; // hit information when there is a hit.
    if (obj && obj.series && obj.series.sub) {
      return basic(container, obj.series.sub);
    } else {
      return basic(container, bigData);
    }

  });

})(document.getElementById("example"), bigData);