Accepted answer

Highcharts wants time values as timestamps in milliseconds instead of Date objects:

What format does the highcharts js library accept for dates?
Highcharts X-Axis time from JS date

Change how you generate your x values, and the zooming magically starts working again:

var x = (new Date()).getTime() + i * frequency,


var x = dataSet[dataSet.length - 1].x + frequency,

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