Overlapping multiline axis labels

HTML

<script src="http://code.highcharts.com/highcharts.js"></script>
<script src="http://code.highcharts.com/modules/exporting.js"></script>
<div id="container" style="width: 800px; height: 400px; margin: 0 auto"></div>

JavaScript

$(function () {
    $('#container').highcharts({
        title: {
            text: 'Monthly Average Temperature',
            x: -20 //center
        },
        subtitle: {
            text: 'Source: WorldClimate.com',
            x: -20
        },
        xAxis: {
            categories: ['Jan 121', 'Feb 2013', 'Mar 2013', 'Apr 2013', 'May 2013', 'Jun 2013', ],
            labels: {
                //y: 30,
                style: {
                    fontSize: '40px'
                    //,lineHeight:'30px'
                }
            }
        },
  
        series: [{
            name: 'Tokyo',
            data: [7.0, 6.9, 9.5, 14.5, 18.2, 21.5]
        }]
    });
});