An ExtJS 4.2 feature extension for the summary feature. This new code adds click events for the summary.

ClickableSummary.js

Ext.define('Mancini.extension.grid.feature.ClickableSummary', {
  extend: 'Ext.grid.feature.Summary',
  alias: ['feature.clickablesummary'],
	
  mixins: {
    observable: 'Ext.util.Observable'
  },
	
  relayedEvents: [
    'summaryclick'
  ],
	
  constructor: function(config) {
    var me = this;
    me.addEvents(
      'summaryclick'
   );
		
    me.callParent(arguments);
    me.mixins.observable.constructor.call(me);
  },
	
  init: function(grid) {
	  var me = this;
		
	  // wire click event to dom cell click           
    grid.on({
      afterrender: function () {
        this.mon(this.el, 'click', me.onSummaryCellClick, me, { delegate: '.x-grid-cell' });
      }                
    });
		
    grid.relayEvents(me, me.relayedEvents);        
    me.callParent(arguments); 
	},
	
  onSummaryCellClick: function (e, target) {            
    var me = this,
        cell = Ext.get(target),
        row = cell.up(me.summaryRowSelector);
        
    // only execute if this was inside summary row
    // as mentioned in init, there may be a cleaner way to implement this
    if (row) {                
      var grid = Ext.getCmp(cell.up('.x-grid').id),
          column = grid.columns[cell.dom.cellIndex],
          record = me.summaryRecord,
		  cellIndex = cell.dom.cellIndex,
		  rowIndex = -1;				
          
      me.fireEvent('summaryclick', me, cell, cellIndex, record, row, rowIndex, e);
    }
  },          
	
	renderTFoot: function(values, out) {
    var view = values.view,
        me = view.findFeature('clickablesummary');

    if (me.showSummaryRow) {
      out.push('<tfoot>');
      me.outputSummaryRecord(me.createSummaryRecord(view), values, out);
      out.push('</tfoot>');
    }
  }	
});