Marking out weekends on a date-based chart

HTML

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

CSS

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

JavaScript

var chart;
var chartData = [];
var chartCursor;

// 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 visits = Math.round(Math.random() * 40) - 20;

        chartData.push({
            date: newDate,
            visits: visits
        });
    }
}

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

    // SERIAL CHART    
    chart = new AmCharts.AmSerialChart();
    chart.autoMarginOffset = 5;
    chart.marginBottom = 0;
    chart.pathToImages = "http://www.amcharts.com/lib/images/";
    chart.zoomOutButton = {
        backgroundColor: '#000000',
        backgroundAlpha: 0.15
    };
    chart.dataProvider = chartData;
    chart.categoryField = "date";
    chart.balloon.bulletSize = 5;

    // listen for "dataUpdated" event (fired when chart is rendered) and call zoomChart method when it happens
    chart.addListener("dataUpdated", zoomChart);

    // AXES
    // category
    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.dashLength = 1;
    categoryAxis.gridAlpha = 0.15;
    categoryAxis.position = "top";
    categoryAxis.axisColor = "#DADADA";

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

    // GRAPH
    var graph = new AmCharts.AmGraph();
    graph.title = "red line";
    graph.valueField = "visits";
    graph.bullet = "round";
    graph.bulletBorderColor = "#FFFFFF";
    graph.bulletBorderThickness = 2;
    graph.lineThickness = 2;
    graph.lineColor =...