highchart x axis not showing correct date labels

i am using highchart and loading a json dynamically. json provides me the date time series and its value

The chart is being created and the tooltip is showing the correct value only the x-axis labels are not shown correctly.

$(function () {
    $('#container').highcharts({
        xAxis: {
            type: 'datetime',
            dateTimeLabelFormats: {
                day: '%e of %b'
            }
        },

        series: [{
            data: [
["2013-09-15 08:44:37",19.8],
["2013-09-15 08:47:37",18.4],
["2013-09-15 08:50:37",18.3],
["2013-09-15 08:53:37",18.1]
],
            //pointStart: Date.UTC(2010, 0, 1),
            pointInterval: 24 * 3600 * 1000 // one day
        }]
    });
});

live plunkr : http://ift.tt/1EArt0e

Kindly assist. Thanks