Updating balloon text with balloonFunction

HTML

<script src="http://www.amcharts.com/lib/3/amcharts.js"></script>
<script src="http://www.amcharts.com/lib/3/serial.js"></script>
<div id="chartdiv" style="width: 100%; height: 362px;"></div>

JavaScript

// since v3, chart can accept data in JSON format
// if your category axis parses dates, you should only
// set date format of your data (dataDateFormat property of AmSerialChart)
var chartData = [
    {
        "lineColor": "#ffc321",
        "date": "2012-01-01",
        "duration": 408
    },
    {
        "date": "2012-01-02",
        "duration": 482
    },
    {
        "date": "2012-01-03",
        "duration": 562
    },
    {
        "date": "2012-01-04",
        "duration": 379
    },
    {
        "lineColor": "#fd813c",
        "date": "2012-01-05",
        "duration": 501
    },
    {
        "date": "2012-01-06",
        "duration": 443
    },
    {
        "date": "2012-01-07",
        "duration": 405
    },
    {
        "date": "2012-01-08",
        "duration": 309,
        "lineColor": "#CC0000"
    },
    {
        "date": "2012-01-09",
        "duration": 287
    },
    {
        "date": "2012-01-10",
        "duration": 485
    },
    {
        "date": "2012-01-11",
        "duration": 890
    },
    {
        "date": "2012-01-12",
        "duration": 810
    }
];
var chart;

AmCharts.ready(function () {
    // SERIAL CHART
    chart = new AmCharts.AmSerialChart();
    chart.dataProvider = chartData;
    chart.categoryField = "date";
    chart.dataDateFormat = "YYYY-MM-DD";
    chart.marginTop = 0;
    
    var balloon = chart.balloon;
    balloon.cornerRadius = 6;
    balloon.adjustBorderColor = false;
    balloon.horizontalPadding = 10;
    balloon.verticalPadding = 10;
    
    // AXES
    // category axis
    var categoryAxis = chart.categoryAxis;
    categoryAxis.parseDates = true; // as our data is date-based, we set parseDates to true
    categoryAxis.minPeriod = "DD"; // our data is daily, so we set minPeriod to DD                
    categoryAxis.autoGridCount = false;
    categoryAxis.gridCount = 50;
    categoryAxis.gridAlpha = 0;
    categoryAxis.gridColor = "#000000";
    categoryAxis.axisColor = "#555555";
    // we want custom...