title: how to show grouped value in Extjs 4 grid groupHeader?
body: <p>I have a grid like this</p>
  
  <pre><code>var groupingFeature = Ext.create('Ext.grid.feature.Grouping', {
      groupHeaderTpl: 'Invoice: {invoice_number} ({rows.length} Item{[values.rows.length &gt; 1 ? "s" : ""]})'
  });
  
  {
  xtype   : 'grid',
  store   : Ext.getStore('invoice_items'),
  columns : [
              { text: 'Invoice Number', flex: 1, dataIndex: 'invoice_number', hidden:true },
              { text: 'Item Code', flex: 1, dataIndex: 'item_code'},
              { text: 'Description', flex: 1.5, dataIndex: 'description' }
          ],
  features: [groupingFeature]                                 
  }
  </code></pre>
  
  <p>im using hidden filed invoice_number to group this grid and i have the output like this</p>
  
  <p><img src="https://i.stack.imgur.com/EUM36.jpg" alt="invoice"></p>
  
  <p>what i want to do is show the grouped invoice number in the group header. is it possible in extjs4? how to do it?</p>
  
  <p>Regards</p>