Chart.js/docs/samples/advanced/linear-gradient.md

Linear Gradient

// <block:actions:3>
const actions = [
  {
    name: 'Randomize',
    handler(chart) {
      chart.data.datasets.forEach(dataset => {
        dataset.data = Utils.numbers({count: chart.data.labels.length, min: -100, max: 100});
      });
      chart.update();
    }
  },
  {
    name: 'Add Data',
    handler(chart) {
      const data = chart.data;
      if (data.datasets.length > 0) {
        data.labels = Utils.months({count: data.labels.length + 1});

        for (var index = 0; index < data.datasets.length; ++index) {
          data.datasets[index].data.push(Utils.rand(-100, 100));
        }

        chart.update();
      }
    }
  },
  {
    name: 'Remove Data',
    handler(chart) {
      chart.data.labels.splice(-1, 1); // remove the label first

      chart.data.datasets.forEach(dataset => {
        dataset.data.pop();
      });

      chart.update();
    }
  }
];
// </block:actions>

// <block:getGradient:0>
let width, height, gradient;
function getGradient(ctx, chartArea) {
  const chartWidth = chartArea.right - chartArea.left;
  const chartHeight = chartArea.bottom - chartArea.top;
  if (gradient === null || width !== chartWidth || height !== chartHeight) {
    // Create the gradient because this is either the first render
    // or the size of the chart has changed
    width = chartWidth;
    height = chartHeight;
    gradient = ctx.createLinearGradient(0, chartArea.bottom, 0, chartArea.top);
    gradient.addColorStop(0, Utils.CHART_COLORS.blue);
    gradient.addColorStop(0.5, Utils.CHART_COLORS.yellow);
    gradient.addColorStop(1, Utils.CHART_COLORS.red);
  }

  return gradient;
}
// </block:getGradient>

// <block:setup:2>
const DATA_COUNT = 7;
const NUMBER_CFG = {count: DATA_COUNT, min: -100, max: 100};
const labels = Utils.months({count: 7});

const data = {
  labels: labels,
  datasets: [
    {
      label: 'Dataset 1',
      data: Utils.numbers(NUMBER_CFG),
      borderColor: function(context) {
        const chart = context.chart;
        const {ctx, chartArea} = chart;

        if (!chartArea) {
          // This case happens on initial chart load
          return null;
        }
        return getGradient(ctx, chartArea);
      },
    },
  ]
};
// </block:setup>

// <block:config:1>
const config = {
  type: 'line',
  data: data,
  options: {
    responsive: true,
    plugins: {
      legend: {
        position: 'top',
      },
    }
  },
};
// </block:config>

module.exports = {
  actions: actions,
  config: config,
};