Stacked Area in JavaScript

// Deterministic daily traffic per channel; stacking shows total site visits
// split into its sources. Each channel is a mean-reverting walk around its
// own base so the composition shifts organically day to day.
function mulberry32(seed) {
  return function () {
    seed |= 0
    seed = (seed + 0x6d2b79f5) | 0
    var t = Math.imul(seed ^ (seed >>> 15), 1 | seed)
    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296
  }
}

function channel(seed, base, spread) {
  var rand = mulberry32(seed)
  var series = []
  var ts = new Date('01 Jun 2025 GMT').getTime()
  var val = base
  for (var i = 0; i < 20; i++) {
    val += (rand() - 0.5) * spread
    val += (base - val) * 0.15
    series.push([ts, Math.max(2, Math.round(val))])
    ts += 86400000
  }
  return series
}

var options = {
  series: [
    {
      name: 'Organic Search',
      data: channel(11, 42, 14),
    },
    {
      name: 'Direct',
      data: channel(23, 26, 9),
    },
    {
      name: 'Social',
      data: channel(35, 15, 8),
    },
  ],
  chart: {
    type: 'area',
    height: 350,
    stacked: true,
    events: {
      selection: function (chart, e) {
        console.log(new Date(e.xaxis.min))
      },
    },
  },
  colors: ['#008FFB', '#00E396', '#FEB019'],
  dataLabels: {
    enabled: false,
  },
  stroke: {
    curve: 'monotoneCubic',
  },
  title: {
    text: 'Website Visits by Channel',
    align: 'left',
  },
  fill: {
    type: 'gradient',
    gradient: {
      opacityFrom: 0.6,
      opacityTo: 0.8,
    },
  },
  legend: {
    position: 'top',
    horizontalAlign: 'left',
  },
  yaxis: {
    labels: {
      formatter: function (val) {
        return val + 'k'
      },
    },
  },
  xaxis: {
    type: 'datetime',
  },
}

var chart = new ApexCharts(document.querySelector('#chart'), options)
chart.render()