highcharts x軸中文刻度太長換行

xAxis: {
    type: 'category',
    title:null,
    gridLineWidth: 1,
    lineColor: "#50ae93",
    labels: {
        style: {
            fontSize: '10px'
        },
        formatter: function() {
            var labelVal = this.value;
            var reallyVal = labelVal;
            var lvl = labelVal.length;
            if(lvl > 8 && lvl <=16){
                reallyVal = labelVal.substr(0,8)+"<br/>"+labelVal.substring(8,lvl);
            }else if(lvl > 16 && lvl <=24){
                reallyVal = labelVal.substr(0,8)+"<br/>"+labelVal.substr(8,8)+"<br/>"+labelVal.substring(16,lvl);
            }else if(lvl > 24){
                reallyVal = labelVal.substr(0,8)+"<br/>"+labelVal.substr(8,8)+"<br/>"+labelVal.substr(16,8)+"<br/>"+labelVal.substring(24,lvl);
            }
            
            return reallyVal;
        }
    },
    categories:['測試2016到2017學年度測試2016到2017學年度測試2016到2017學年度','測試2016到2017學年度測試2016到2017學年度測試2016到2017學年度','測試2016到2017學年度測試2016到2017學年度測試2016到2017學年度']
},