Chartjs-How to remove transparency from images downloaded in Chart.js

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No by default canvases dont have a background so it will always be transparent. To fix this you can write a small custom plugin that draws a white background before chart.js draws its things on it:

Chart.register({
  id: 'customBackground',
  beforeDraw: (chart, args, opts) => {
    const ctx = chart.canvas.getContext('2d');
    ctx.save();
    ctx.globalCompositeOperation = 'destination-over';
    ctx.fillStyle = 'white';
    ctx.fillRect(0, 0, chart.width, chart.height);
    ctx.restore();
  }
})

const options = {
  type: 'line',
  data: {
    labels: ["Red", "Blue", "Yellow", "Green", "Purple", "Orange"],
    datasets: [{
        label: '# of Votes',
        data: [12, 19, 3, 5, 2, 3],
        borderColor: 'orange'
      },
      {
        label: '# of Points',
        data: [7, 11, 5, 8, 3, 7],
        borderColor: 'pink'
      }
    ]
  },
  options: {},
}

const ctx = document.getElementById('chartJSContainer').getContext('2d');
new Chart(ctx, options);
body {
  background-color: black;
}
<body>
  <canvas id="chartJSContainer" width="600" height="400"></canvas>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/3.6.2/chart.js"></script>
</body>

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