Showing indicator while chart is loading

HTML

<script src="http://www.amcharts.com/lib/3/amcharts.js" type="text/javascript"></script>
<script src="http://www.amcharts.com/lib/3/serial.js" type="text/javascript"></script>
<div id="chartdiv" style="width: 400px; height: 300px; border: 1px solid #ccc;"></div>

CSS

body
{
    font-size:12px;
    color:#000000;
    background-color:#ffffff;
    font-family:verdana,helvetica,arial,sans-serif;
}

JavaScript

var chart;
var chartData = [];

// generate some random data, quite different range
function generateChartData() {
    var firstDate = new Date();
    firstDate.setDate(firstDate.getDate() - 500);

    for (var i = 0; i < 500; i++) {
        var newDate = new Date(firstDate);
        newDate.setDate(newDate.getDate() + i);
        
        var dataPoint = {
            date: newDate
        };
        
        // generate values for 50 graphs
        for (var c = 0; c < 100; c++) {
            dataPoint['value'+c] = Math.round(Math.random() * 40) - 20;
        }

        chartData.push(dataPoint);
    }
}

// creat chart
AmCharts.ready(function() {
    // generate some data
    generateChartData();

    // SERIAL CHART    
    chart = new AmCharts.AmSerialChart();
    chart.pathToImages = "http://www.amcharts.com/lib/3/images/";
    chart.dataProvider = chartData;
    chart.categoryField = "date";


    // AXES
    // category
    var categoryAxis = chart.categoryAxis;
    categoryAxis.labelsEnabled = false;
    categoryAxis.ignoreAxisWidth = true;
    categoryAxis.axisAlpha = 0;

    // value                
    var valueAxis = new AmCharts.ValueAxis();
    valueAxis.axisAlpha = 0;
    valueAxis.dashLength = 1;
    chart.addValueAxis(valueAxis);

    // GRAPH
    for (var c = 0; c < 100; c++) {
        var graph = new AmCharts.AmGraph();
        graph.valueField = "value"+c;
        graph.bullet = "round";
        graph.bulletBorderColor = "#FFFFFF";
        graph.bulletBorderThickness = 2;
        graph.lineThickness = 2;
        graph.lineAlpha = 0.5;
        graph.fillAlphas = 0.5;
        graph.hideBulletsCount = 50; // this makes the chart to hide bullets when there are more than 50 series in selection
        chart.addGraph(graph);
    }

    // WRITE
    chart.write("chartdiv");
});