title: dc.js group.top(5) not working in chart
body: <p>i am trying to display top 5 country based on revenue</p>
  
  <p>i tried feeding in chart its not working</p>
  
  <pre><code>.group(countryGrp.top(10))
  </code></pre>
  
  <p>but when i tried <code>console.log(countryGrp.top(10))</code> i can see the values though</p>
  
  <pre><code>var country = data.dimension(function(d){return d.PName});
  var countryGrp = lead.group().reduceSum(function(d) {
              return d.Amount;
          });
  
  var leadBarChart = dc.barChart("#country-chart")
          leadBarChart.width(500).height(450).transitionDuration(750)
                  .margins({
                      top : 20,
                      right : 10,
                      bottom : 80,
                      left : 50
                  })
                  .dimension(country)
                  .group(countryGrp.top(10))
                  .ordinalColors([ "#1F77B4" ]).elasticY(true).centerBar(true)
                  .title(function(d) {
                      return d.Amount;
                  }).gap(6).xUnits(function() {
                      return 2;
                  })
                  .x(d3.scale.linear())
                  .renderHorizontalGridLines(true).xAxis().ticks(5)
                  .tickFormat(d3.format("d"));
  </code></pre>
  
  <p>i am getting this error uncaught TypeError: undefined is not a function  can any one help me out here. Thanks in advance</p>