Chartjs-How to export Chart.js chart using toBase64Image, but with no transparency?

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This behaviour comes because there is nothing drawn on the chart as a background. This can be fixed with a custom inline plugin like so (fillStyle is the color of the background):

 plugins: [{
        id: 'custom_canvas_background_color',
        beforeDraw: (chart) => {
          const ctx = chart.canvas.getContext('2d');
          ctx.save();
          ctx.globalCompositeOperation = 'destination-over';
          ctx.fillStyle = 'white';
          ctx.fillRect(0, 0, chart.canvas.width, chart.canvas.height);
          ctx.restore();
        }
      }]

example:

var options = {
  type: 'line',
  data: {
    labels: ["Red", "Blue", "Yellow", "Green", "Purple", "Orange"],
    datasets: [{
        label: '# of Votes',
        data: [12, 19, 3, 5, 2, 3],
        borderWidth: 1
      },
      {
        label: '# of Points',
        data: [7, 11, 5, 8, 3, 7],
        borderWidth: 1
      }
    ]
  },
  options: {
    scales: {
      yAxes: [{
        ticks: {
          reverse: false
        }
      }]
    }
  },
  plugins: [{
    id: 'custom_canvas_background_color',
    beforeDraw: (chart) => {
      const ctx = chart.canvas.getContext('2d');
      ctx.save();
      ctx.globalCompositeOperation = 'destination-over';
      ctx.fillStyle = 'white';
      ctx.fillRect(0, 0, chart.canvas.width, chart.canvas.height);
      ctx.restore();
    }
  }]
}

var ctx = document.getElementById('chartJSContainer').getContext('2d');
new Chart(ctx, options);
<body>
  <canvas id="chartJSContainer" width="600" height="400"></canvas>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.4/Chart.js" integrity="sha512-hZf9Qhp3rlDJBvAKvmiG+goaaKRZA6LKUO35oK6EsM0/kjPK32Yw7URqrq3Q+Nvbbt8Usss+IekL7CRn83dYmw==" crossorigin="anonymous"></script>
</body>

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