Highchart add series

The series is made by differencing the two series percentage change from first point

HTML

<script src="https://code.highcharts.com/highcharts.js"></script>
<script src="https://code.highcharts.com/modules/exporting.js"></script>

<div id="container" style="min-width: 310px; height: 400px; margin: 0 auto"></div>

JavaScript

function add() {
  var chart = this;
  var newseriesdata = [];
  newseriesdata.push(0);
  for (var x = 1; x < chart.series[0].points.length; x++) {
    var diff0 = (chart.series[0].points[x].y - chart.series[0].points[0].y) / chart.series[0].points[0].y;
    var diff1 = (chart.series[1].points[x].y - chart.series[1].points[0].y) / chart.series[1].points[0].y;
    newseriesdata.push(100 * (diff1 - diff0));

  }
  chart.addAxis({ // Secondary yAxis
    title: {
      text: 'Diff %'
    },
    lineWidth: 2,
    lineColor: '#CCC',
    plotBands: [{
      from: 0,
      to: 100,
      color: 'rgba(0,255, 0, 0.02)',
      label: {
        text: 'Uplift'
      }
    }, {
      from: -100,
      to: 0,
      color: 'rgba(255, 0, 0, 0.02)',
      label: {
        text: 'Drop'
      }
    }]
  });
  chart.addSeries({
    name: "Diff",
    type: 'spline',
    data: newseriesdata,
    dashStyle: 'dash',
    color: "#000",
    yAxis: 3
  });

  var ext = chart.yAxis[1].getExtremes();
  var mx = 0;
  mx = (ext.dataMax > -ext.dataMin) ? ext.dataMax : -ext.dataMin;
  chart.yAxis[1].setExtremes(-mx, mx);
  chart.series[0].hide();
  chart.series[1].hide();
  chart.yAxis[0].update({
    labels: {
      enabled: false
    },
    title: {
      text: null
    }
  });
}





$(function() {
  $('#container').highcharts({
    chart: {
      type: 'spline',
      events: {
        load: add
      }
    },
    title: {
      text: 'Sales'
    },
    xAxis: {
      allowDecimals: false,
      labels: {
        formatter: function() {
          return this.value; // clean, unformatted number for year
        }
      }
    },
    yAxis: {
      title: {
        text: 'Sales'
      },
      min: 0
    },
    tooltip: {
      pointFormat: '{series.name} produced <b>{point.y:,.0f}</b><br/>warheads in {point.x}'
    },
    plotOptions: {
      spline: {
        cpointStart: 1940,
        xlineWidth: 3,
        xdashStyle: 'dash'
      }
    },
    series: [{
      name: 'Test',
      type:...