Refresh summary in an Ext4 grid

HTML

<link rel="stylesheet" href="/js/lib/ext-4.0.2a/resources/css/ext-all.css">

JavaScript

Ext.require('Ext.grid.feature.Summary', function() {
        Ext.override(Ext.grid.feature.Summary, {
            refresh: function() {
                if(this.view.rendered) {
                    var tpl = Ext.create(
                                'Ext.XTemplate',
                                '{[this.printSummaryRow()]}',
                                this.getFragmentTpl()
                              );
                    tpl.overwrite(this.getRowEl(), {});
                }
            },

            getRowEl: function() {
                return this.view.el.down('tr.x-grid-row-summary');
            }
        });
    });
    Ext.onReady(function() {
        var sm = Ext.create('Ext.selection.CheckboxModel', {
            listeners:{
                selectionchange: function(selectionModel, selectedRecords, options) {
                    grid.getView().getFeature(0).refresh();
                }
            }
        });

        var selSumFn = function(column, selModel) {
            return function() {
                var records = selModel.getSelection(),
                        result = 0;
                Ext.each(records, function(record) {
                    result += record.get(column) * 1;
                });

                return result;
            };
        };


        var store = Ext.create('Ext.data.ArrayStore', {
            expandData: true,
            // store configs
            autoDestroy: true,
            storeId: 'myStore',
            // reader configs
            idIndex: 0,
            data: [[0, 1, 2], [1, 3, 4], [2, 5, 6], [3, 7, 8], [4, 9, 10]],
            fields: [
                {name: 'id', type: 'int'},
                {name: 'col1', type: 'int'},
                {name: 'col2', type: 'int'},
            ]
        });

        var grid = Ext.create('Ext.grid.Panel', {
            renderTo: Ext.getBody(),
            autoScroll:true,
            features: [
                {
                    ftype: 'summary'
  ...