title: Extjs 4: pie chart display groupField from store
body: <p>I'm looking to create a pie chart that displays a group field from a store. For example, if each student in my store has a 'grade' field, I want the pie chart to show the relative amount of students in each grade. Is this possible? Or does the pie chart have to display a field
  My store:</p>
  
  <pre><code>Ext.define('AM.store.Users', {
      extend: 'Ext.data.Store',
      model: 'AM.model.User',
      data: [
          {Name: 'derp', Grade: 'FR'},
          {Name: 'herp', Grade: 'SR'},
                   ....
          {Name: 'beavis', Grade: 'SR'}
      ],
      groupField: 'Grade',
   });
  </code></pre>
  
  <p>My attempt at a chart:</p>
  
  <pre><code>Ext.define('AM.view.user.Chart' , {
      extend: 'Ext.chart.Chart',
      alias: 'widget.userchart',
      width: 500,
      height: 600,
      animate: true,
      store: 'Users',
      shadow: true,
      legend: { position: 'right'},
      insetPadding: 25,
      series: [{
              type: 'pie',
              angleField: 'PreReq',
              showInLegend: true,
              highlight: {
                      segment: {
                              margin: 20
                      }
              }
              animate: true
      }]
  });
  </code></pre>
  
  <p>thanks for any tips!</p>