highcharts random x dates

by grant

HTML

<script src="https://code.highcharts.com/highcharts.js"></script>
<script src="https://code.highcharts.com/modules/exporting.js"></script>
<script src="https://code.highcharts.com/modules/series-label.js"></script>
<div id="container" style="min-width: 310px; height: 400px; margin: 0 auto">
</div>

JavaScript

Highcharts.chart('container', {
  chart: {
    type: 'spline'
  },
  title: {
    text: 'Snow depth at Vikjafjellet, Norway'
  },
  subtitle: {
    text: 'Irregular time data in Highcharts JS'
  },
  xAxis: {
    type: 'datetime',
    dateTimeLabelFormats: { // don't display the dummy year
      month: '%e. %b',
      year: '%b'
    },
    title: {
      text: 'Date'
    }
  },
  yAxis: {
    title: {
      text: 'Snow depth (m)'
    },
    min: 0
  },
  tooltip: {
    headerFormat: '<b>{series.name}</b><br>',
    pointFormat: '{point.x:%e. %b}: {point.y:.2f} m'
  },

  plotOptions: {
    spline: {
      marker: {
        enabled: true
      }
    }
  },

  series: [{
    name: 'Winter 2012-2013',
    // Define the data points. All series have a dummy year
    // of 1970/71 in order to be compared on the same x axis. Note
    // that in JavaScript, months start at 0 for January, 1 for February etc.
    data: [
      [Date.UTC(1970, 9, 21), 0],
      [Date.UTC(1970, 10, 4), 0.28],
      [Date.UTC(1970, 10, 9), 0.25],
      [Date.UTC(1970, 10, 27), 0.2],
      [Date.UTC(1970, 11, 2), 0.28],
      [Date.UTC(1970, 11, 26), 0.28],
      [Date.UTC(1970, 11, 29), 0.47],
      [Date.UTC(1971, 0, 11), 0.79],
      [Date.UTC(1971, 0, 26), 0.72],
      [Date.UTC(1971, 1, 3), 1.02],
      [Date.UTC(1971, 1, 11), 1.12],
      [Date.UTC(1971, 1, 25), 1.2],
      [Date.UTC(1971, 2, 11), 1.18],
      [Date.UTC(1971, 3, 11), 1.19],
      [Date.UTC(1971, 4, 1), 1.85],
      [Date.UTC(1971, 4, 5), 2.22],
      [Date.UTC(1971, 4, 19), 1.15],
      [Date.UTC(1971, 5, 3), 0]
    ]
  }, {
    name: 'Winter 2013-2014',
    data: [
      [Date.UTC(1970, 9, 29), 0],
      [Date.UTC(1970, 10, 9), 0.4],
      [Date.UTC(1970, 11, 1), 0.25],
      [Date.UTC(1971, 0, 1), 1.66],
      [Date.UTC(1971, 0, 10), 1.8],
      [Date.UTC(1971, 1, 19), 1.76],
      [Date.UTC(1971, 2, 25), 2.62],
      [Date.UTC(1971, 3, 19), 2.41],
      [Date.UTC(1971, 3, 30), 2.05],
      [Date.UTC(1971, 4, 14),...