Chart.js Line, different fill color for negative point

To get @potatopeelings code above to work with chart.js 2.5.x you need to add yAxisID : 'y-axis-0' into your datasets, as below.

var myLineChart = new Chart(ctx, {
  type: 'NegativeTransparentLine',
  data: {
    labels: ["January", "February", "March", "April", "May", "June", "July"],
    datasets: [{
      yAxisID : 'y-axis-0',
      ....

i update the method to work with multiple datasets.

Chart.defaults.NegativeTransparentLine = Chart.helpers.clone(Chart.defaults.line);
    Chart.controllers.NegativeTransparentLine = Chart.controllers.line.extend({
        update: function () {
          for(let i=0; i< this.chart.data.datasets.length; i++) {
            // get the min and max values
            var min = Math.min.apply(null, this.chart.data.datasets[i].data);
            var max = Math.max.apply(null, this.chart.data.datasets[i].data);
            var yScale = this.getScaleForId(this.chart.data.datasets[i].yAxisID);

            // figure out the pixels for these and the value 0
            var top = yScale.getPixelForValue(max);
            var zero = yScale.getPixelForValue(0);
            var bottom = yScale.getPixelForValue(min);

            // build a gradient that switches color at the 0 point
            var ctx = this.chart.chart.ctx;
            var gradient = ctx.createLinearGradient(0, top, 0, bottom);
            var ratio = Math.min((zero - top) / (bottom - top), 1);
            gradient.addColorStop(0, 'rgba(55,210,99,0.4)');
            gradient.addColorStop(ratio, 'rgba(55,210,99,0.4)');
            gradient.addColorStop(ratio, 'rgba(247,100,120,0.4)');
            gradient.addColorStop(1, 'rgba(247,100,120,0.4)');
            this.chart.data.datasets[i].backgroundColor = gradient;
          }
          return Chart.controllers.line.prototype.update.apply(this, arguments);
        }
    });

Tested on chart.js 2.8.0 on Angular 8

import { Component, OnInit, ViewChild } from '@angular/core';
import { Chart, ChartDataSets, ChartOptions } from 'chart.js';
import { Color, Label } from 'ng2-charts';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  public lineChartData: ChartDataSets[] = [
    { data: [89, 0, -80, 81, 56, -55, 40], label: 'Series A', yAxisID: 'y-axis-0' },
    { data: [-890, 0, 800, -810, -560, 550, -400], label: 'Series B', yAxisID: 'y-axis-0' },
  ];
  public lineChartLabels: Label[] = ['January', 'February', 'March', 'April', 'May', 'June', 'July'];
  public lineChartOptions: (ChartOptions & { annotation: any }) = {
    responsive: true,
  };
  public lineChartColors: Color[] = [
    {
      backgroundColor: 'rgba(255,0,0,0.3)',
    },
    {
      backgroundColor: 'rgba(0,255,0,0.3)',
    },
  ];
  public lineChartLegend = true;
  public lineChartType = 'line';
  public lineChartPlugins = [];

  constructor() {
    Chart.defaults.NegativeTransparentLine = Chart.helpers.clone(Chart.defaults.line);
    Chart.controllers.NegativeTransparentLine = Chart.controllers.line.extend({
        update: function () {
          for(let i=0; i< this.chart.data.datasets.length; i++) {
            // get the min and max values
            var min = Math.min.apply(null, this.chart.data.datasets[i].data);
            var max = Math.max.apply(null, this.chart.data.datasets[i].data);
            var yScale = this.getScaleForId(this.chart.data.datasets[i].yAxisID);

            // figure out the pixels for these and the value 0
            var top = yScale.getPixelForValue(max);
            var zero = yScale.getPixelForValue(0);
            var bottom = yScale.getPixelForValue(min);

            // build a gradient that switches color at the 0 point
            var ctx = this.chart.chart.ctx;
            var gradient = ctx.createLinearGradient(0, top, 0, bottom);
            var ratio = Math.min((zero - top) / (bottom - top), 1);
            gradient.addColorStop(0, 'rgba(55,210,99,0.4)');
            gradient.addColorStop(ratio, 'rgba(55,210,99,0.4)');
            gradient.addColorStop(ratio, 'rgba(247,100,120,0.4)');
            gradient.addColorStop(1, 'rgba(247,100,120,0.4)');
            this.chart.data.datasets[i].backgroundColor = gradient;
          }
          return Chart.controllers.line.prototype.update.apply(this, arguments);
        }
    });
    this.lineChartType = 'NegativeTransparentLine';
  }

  ngOnInit() {
  }
}

<div style="display: block;">
  <canvas baseChart width="400" height="400"
    [datasets]="lineChartData"
    [labels]="lineChartLabels"
    [options]="lineChartOptions"
    [colors]="lineChartColors"
    [legend]="lineChartLegend"
    [chartType]="lineChartType"
    [plugins]="lineChartPlugins">
  </canvas>
</div>

You can extend the line chart to do this.

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Preview

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Script

Chart.defaults.NegativeTransparentLine = Chart.helpers.clone(Chart.defaults.line);
Chart.controllers.NegativeTransparentLine = Chart.controllers.line.extend({
    update: function () {
        // get the min and max values
        var min = Math.min.apply(null, this.chart.data.datasets[0].data);
        var max = Math.max.apply(null, this.chart.data.datasets[0].data);
        var yScale = this.getScaleForId(this.getDataset().yAxisID);

        // figure out the pixels for these and the value 0
        var top = yScale.getPixelForValue(max);
        var zero = yScale.getPixelForValue(0);
        var bottom = yScale.getPixelForValue(min);

        // build a gradient that switches color at the 0 point
        var ctx = this.chart.chart.ctx;
        var gradient = ctx.createLinearGradient(0, top, 0, bottom);
        var ratio = Math.min((zero - top) / (bottom - top), 1);
        gradient.addColorStop(0, 'rgba(75,192,192,0.4)');
        gradient.addColorStop(ratio, 'rgba(75,192,192,0.4)');
        gradient.addColorStop(ratio, 'rgba(0,0,0,0)');
        gradient.addColorStop(1, 'rgba(0,0,0,0)');
        this.chart.data.datasets[0].backgroundColor = gradient;

        return Chart.controllers.line.prototype.update.apply(this, arguments);
    }
});

and then

 ...
 var myLineChart = new Chart(ctx, {
     type: 'NegativeTransparentLine',
     data: {
     ...

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Fiddle - http://jsfiddle.net/g2r2q5Lu/