I am trying to capture the exact series name from the item click on a line chart my code looks like the following

Code:
function showChart() {
   var store = Ext.data.JsonStore{ (
        fields: [ 'date', 'sales', 'rev' ],
        url: 'chartRender.jsp'
        root: 'data',
        autoLoad: true
   } );

  var chartPanel = new Ext.Panel ( {
       title: 'Sales and Revenue Chart'
       renderTo: 'container',
       width: 1000,
       height: 500,
       layout: 'fit',
     
       items: {
         xtype: 'linechart',
         store: store,
         xField: 'date',
     
         xAxis: new Ext.chart.CategoryAxis( {
                  title: 'date'
         } ),
         yAxis: new Ext.chart.NumericAxis ( {
                 title: 'USD'
         } ),
 
         series: [ {
             yField: 'sales',
              displayName: 'Sales'
         }, { 
             yField: 'rev',
             displayName: 'Revenue'
         } ],

        listeners: {
             itemclick: function(o) {
                    // HOW DO I GET THE NAME OF THE SERIES THE USER CLICKED?
                    // WHAT IS THE "o" OBJECT BEING PASS IN AND WHAT METHODS OR CONTANTS CAN I CALL ON IT?
                    // THE CODE BELOW ONLY GIVES THE INDEX TO THE STORE BUT DOES NOT SAY WHICH SERIES WAS CLICKED BY THE USER
                    var rec = store.getAt(o.index);
             }

        }
  });
}

Please help the Ext JS Documentation is not very good and I get confused just trying to find what other listeners can I use and how to use them. I have been trying to figure this out for two days with no progress.