xAxis: {
                categories: []

            },

xAxis: {
            labels: {
                formatter: function () {
                    return '<span class="XYZ">' + this.value + '</span>';
                },
                useHTML: true
            }
        }

xAxis: {
   labels: {
      useHTML:true,
      style: {
           width: 50 //where 50 is value in pixels
      }
   }
}