Accepted answer

you can do this on a call and looping through the series[0].data values. since you say you only have one series per chart i am also assuming you only have one xaxis as well. you would loop through your data like so:

var seriesdata = this.series[0].data;
var tcategories = [];
for (i = 0; i < seriesdata.length; i++) {

live demo.

less complex method is to define your xaxis.type as 'category':

"xaxis": {
    "type": "category"

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