title: Change value on legend in dc.js
body: <p>I built a pie chart with dc.js, with the following:</p>
  
  <pre><code>var chart = dc.pieChart(selector, this.chartGroup)
    .width(200).height(200)
    .dimension(this.dimension)
    .group(this.group)
    ...
    .legend(dc.legend().x(10).y(255).gap(5).horizontal(true))
  
  chart.render()
  </code></pre>
  
  <p>Is there a way to format the labels on the legend with something like the following:</p>
  
  <pre><code>function (d) {
    return d.key + ': ' + d.value;
  }
  </code></pre>