Highcharts.chart('container', {
    chart: {
        type: 'area'
    },
    xAxis: {
        categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
    },

    plotOptions: {
        series: {
            fillColor: {
                linearGradient: [0, 0, 0, 300],
                stops: [
                    [0, Highcharts.getOptions().colors[0]],
                    [1, Highcharts.color(Highcharts.getOptions().colors[0]).setOpacity(0).get('rgba')]
                ]
            }
        }
    },

    series: [{
        data: [29.9, 71.5, 106.4, 129.2, 144.0, 176.0, 135.6, 148.5, 216.4, 194.1, 95.6, 54.4]
    }]
});