Automatically handling negative values in pie chart

HTML

<script type="text/javascript" src="http://www.amcharts.com/lib/3/amcharts.js"></script>
<script type="text/javascript" src="http://www.amcharts.com/lib/3/pie.js"></script>
<script type="text/javascript" src="http://www.amcharts.com/lib/3/themes/none.js"></script>
<div id="chartdiv"></div>

CSS

#chartdiv {
    width : 100%;
    height : 500px;
    font-size : 11px;
}

JavaScript

// set initial data with negative values
var chartData = [{
    "country": "Lithuania",
        "litres": 50
}, {
    "country": "Czech Republic",
        "litres": -30
}, {
    "country": "Ireland",
        "litres": 15
}, {
    "country": "Germany",
        "litres": -5
}];

// manipulate the data
for (var i in chartData) {
    chartData[i].absValue = Math.abs(chartData[i].litres);
}
console.log(chartData);
var chart = AmCharts.makeChart("chartdiv", {
    "type": "pie",
        "theme": "none",
        "dataProvider": chartData,
        "valueField": "absValue",
        "titleField": "country",
        "balloonText": "[[title]]: [[percents]]% ([[litres]])",
        "labelText": "[[title]]: [[percents]]% ([[litres]])",
        "exportConfig": {
        menuItems: [{
            icon: '/lib/3/images/export.png',
            format: 'png'
        }]
    }
});