Highcharts.chart('container', {

    title: {
        text: 'Highcharts custom label'
    },

    xAxis: {
        categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
    },

    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]
    }]

}, function (chart) { // on complete
    var point = chart.series[0].points[8];
    chart.renderer.label('Max observation', 270, 50, 'callout', point.plotX + chart.plotLeft, point.plotY + chart.plotTop)
        .css({
            color: '#FFFFFF'
        })
        .attr({
            fill: 'rgba(0, 0, 0, 0.75)',
            padding: 8,
            r: 5,
            zIndex: 6
        })
        .add();

});