Accepted answer

One solution is to use a tickPositioner function and specify every single index:

xAxis: {
    tickPositioner: function() {
        var result = [];
        for(i = 0; i < _mainData.length; i++)
        return result;

See this JSFiddle example. You can remove xAxis.labels.step when using this.

At first tickInterval looks easier, but unfortunately doesn't work because of the following note:

If the tickInterval is too dense for labels to be drawn, Highcharts may remove ticks.

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