score:1

Accepted answer

This is caused by data-grouping feature. You can disable it or define a custom format for each time span.

  series: [{
    data: data,
    dataGrouping: {
      enabled: false,
      ...
    }
  }]

Live demo: https://jsfiddle.net/BlackLabel/yor987mt/

Docs: https://www.highcharts.com/docs/stock/data-grouping

API Reference: https://api.highcharts.com/highstock/series.line.dataGrouping.dateTimeLabelFormats

score:0

It seems there is a workaround using fooltip.formatter which allows to define a tooltip for both the series and the dates using a callback function which returns an array of the form [date, y1, y2, ...].

tooltip: {
    formatter: _ => ['test custom date', 'custom y']
}

In my case, I now reuse the default formatter for the y value but replace the logic to format the dates

tooltip: {
    formatter: function(tooltip) {
         let tt = tooltip.defaultFormatter.call(this, tooltip);
         tt[0] = 'test custom date';
         return tt;
    }
}

Live example: https://jsfiddle.net/GregorDeCillia/5hj6r1ft/58/


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