score:10

The Plugin Core API offers different hooks that may be used for executing custom code. You already use the beforeDraw hook to draw text in the middle of the doughnut.

You could now also use the beforeInit hook to modify the chart configuration in order to fit your needs:

beforeInit: (chart) => {
  const dataset = chart.data.datasets[0];
  chart.data.labels = [dataset.label];
  dataset.data = [dataset.percent, 100 - dataset.percent];
}

Given this code, the definition of your dataset would look simple as follows:

{
  label: 'Africa / Population (millions)',
  percent: 68,
  backgroundColor: ['#5283ff']
}

Last you have to define a tooltips.filter, so that the tooltip appears only at the relevant segment.

tooltips: {
  filter: tooltipItem => tooltipItem.index == 0
}

Please take a look at your amended code and see how it works.

var myChartCircle = new Chart('chartProgress', {
  type: 'doughnut',
  data: {
    datasets: [{
      label: 'Africa / Population (millions)',
      percent: 68,
      backgroundColor: ['#5283ff']
    }]
  },
  plugins: [{
      beforeInit: (chart) => {
        const dataset = chart.data.datasets[0];
        chart.data.labels = [dataset.label];
        dataset.data = [dataset.percent, 100 - dataset.percent];
      }
    },
    {
      beforeDraw: (chart) => {
        var width = chart.chart.width,
          height = chart.chart.height,
          ctx = chart.chart.ctx;
        ctx.restore();
        var fontSize = (height / 150).toFixed(2);
        ctx.font = fontSize + "em sans-serif";
        ctx.fillStyle = "#9b9b9b";
        ctx.textBaseline = "middle";
        var text = chart.data.datasets[0].percent + "%",
          textX = Math.round((width - ctx.measureText(text).width) / 2),
          textY = height / 2;
        ctx.fillText(text, textX, textY);
        ctx.save();
      }
    }
  ],
  options: {
    maintainAspectRatio: false,
    cutoutPercentage: 85,
    rotation: Math.PI / 2,
    legend: {
      display: false,
    },
    tooltips: {
      filter: tooltipItem => tooltipItem.index == 0
    }
  }
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.3/Chart.min.js"></script>
<canvas id="chartProgress"></canvas>


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