ChartJs, How can I get different color fills between my two datasets in a line graph?

Question

Above is a graph I made using ChartJS. I have two datasets. I also have the space between the two datasets filled just like I want it. But I need to get two different colors for the fill. When the Hours dataset is bigger than the Goal dataset, I want the fill to be green, and when the Hours dataset is smaller than Goal, I want it to be red. I'm really hoping there is a way to do this using ChartJS, so that I don't have to recreate this in Canvas.

If it matters, this is in Vue.Js.

Here is the code for the two datasets:

    const dataSets = [{

        label: this.dataSetLabel,
        data: this.dataArray,
        backgroundColor: new Array(this.dataArray.length).fill('rgba(255, 0, 0, 0.8)'), // red
        borderColor: new Array(this.dataArray.length).fill('rgba(255, 0, 0, 0.8)'),
        borderWidth: 1,
        fill: '+1'
      }]
      if (this.fi !== 3) {

        dataSets.push({
          label: 'Goal',
          data: new Array(this.dataArray.length).fill(this.user.braceHours),
          type: 'line',
          backgroundColor: new Array(this.dataArray.length).fill('rgba(84, 232, 198, 0.8)'), // green
          borderColor: new Array(this.dataArray.length).fill('rgba(84, 232, 198, 0.8)'),
          borderWidth: 1,
          fill: '-1'
        })
      }

Any help with this would be much appreciated.

Answer 1

You can extend an existing line chart as shown by the runnable code snippted below.

This solution is based on the answer https://stackoverflow.com/a/36941860/2358409 that had to be slightly adapted to work with the latest stable version of Chart.js (2.9.3).

const goal = 2;
Chart.defaults.areaConditionalColors = Chart.defaults.line;
Chart.controllers.areaConditionalColors = Chart.controllers.line.extend({
  update: function(reset) {
    var yAxis = this.chart.scales['y-axis-0'];  
    var max = Math.max.apply(null, this.chart.data.datasets[0].data);
    var yTop = yAxis.getPixelForValue(max);
    var yGoal = yAxis.getPixelForValue(goal);
    var min = Math.min.apply(null, this.chart.data.datasets[0].data);
    var yBottom = yAxis.getPixelForValue(min);

    // build a gradient that changes the color at the goal
    var ctx = this.chart.chart.ctx;
    var gradient = ctx.createLinearGradient(0, yTop, 0, yBottom);
    var ratio = Math.min((yGoal - yTop) / (yBottom - yTop), 1);
    gradient.addColorStop(0, 'green');
    gradient.addColorStop(ratio, 'green');
    gradient.addColorStop(ratio, 'red');
    gradient.addColorStop(1, 'red');
    this.chart.data.datasets[0].backgroundColor = gradient;

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

new Chart('myChart', {
  type: 'areaConditionalColors',
  data: {
    labels: ['FRI', 'SAT', 'SUN', 'MON', 'TUE', 'WED', 'THU'],
    datasets: [{
      label: 'Hours',
      backgroundColor: 'green',
      data: [0, 3, 0, 9, 4, 0, 0],
      fill: '+1'
    },
    {
      label: 'Goal',
      backgroundColor: 'rgba(84, 232, 198, 0.8)',
      data: [goal, goal, goal, goal, goal, goal, goal],
      fill: '-1'
    }]
  },
  options: {
    legend: {
      onClick: e => e.stopPropagation()
    },
    tooltips: {
      mode: 'x'
    }
  }  
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.3/Chart.min.js"></script>
<canvas id="myChart" height="80"></canvas>