Como alterar dinamicamente a cor da linha com base no valor inferior ao anterior em gráficos altos

series: [{
  name: 'Random data',
  colorByPoint: true,
  data: (function() {
    // generate an array of random data
    var time = (new Date()).getTime(),
      i,
      yValue;

    for (i = -19; i <= 0; i += 1) {
      yValue = Math.random();

      if (i > -19 && yValue > data[data.length - 1].y) { // Green point
        zones.push({
          color: "#5f9",
          value: time + i * 1000,
        });
      } else if (i > -19 && yValue <= data[data.length - 1].y) { // black point
        zones.push({
          color: "#000",
          value: time + i * 1000,
        });
      } else { // first point alway green
        zones.push({
          color: "#5f9",
          value: time + i * 1000,
        });
      }
      data.push({
        x: time + i * 1000,
        y: yValue
      });
    }
    return data;
  }()),
  zoneAxis: "x",
  zones: zones
}]