I have been using HighCharts in my PHP website by migrating it from older charts and I am very impressed by the number of graph options and functions with this library.
It's been a while since I've done work in highcharts, but I believe you just need to provide a formatter function. For example:
xAxis: {
categories: [
'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'
],
labels: {
formatter: function () {
return '' + this.value + ''
},
useHTML: true
}
},