title: extjs summary of column which values has multiplication of previous 2 columns
body: <p>As it has been said in title problem is that summary of last column doesn't work, here is the code:</p>
  
  <pre><code>,{
      id: 'result_id',
      header: " ",
      sortable: true,
      dataIndex: 'result',            
      width: 100,
      renderer: function (value, style, record, row, col, store, grid) {
        return Ext.util.Format.number(record.get('prew1') * record.get('prew2'), '0.00');
      },  
      summaryType: 'sum',
      summaryRenderer: function(v) {
          return  Ext.util.Format.number(v, '0.00');
      }  
  </code></pre>
  
  <p>So multyplication works as expected but sum always stay 0, any thoughts?</p>
  
  <p>I tryed also to specify custom summaryType like this:</p>
  
  <pre><code>summaryType:function (values) {
      var result = [], retval;
  
      Ext.Array.forEach(values, function (record){
          result.push(record.get('area') * record.get('yield'));
      });       
      retval = Ext.Array.mean(result); 
      return Ext.util.Format.number(retval, '0.00');
  },
  </code></pre>
  
  <p>But that couse a ExtJs problem Error: </p>
  
  <blockquote>
    <p>Ext.ux.grid.GroupSummary.Calculations[cf.summaryType] is not a
    function Source File: /lib/ext/ux/GroupSummary.js Line: 124</p>
  </blockquote>
  
  <p>Any ideas?</p>
  
  <p>Thanks</p>