score:1

Accepted answer

There doesn't seem to be any native way to show the percentages, but it's quite straightforward to calculate and set them as the labels:

const data = [1, 2, 3],
  // ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
  total = data.reduce((accumulator, currentValue) => accumulator + currentValue),
  // ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map
  labels = data.map(value => Math.round((value / total) * 100) + '%');

new Chart(document.getElementById('chart'), {
  type: 'doughnut',
  data: {
    labels: labels,
    datasets: [{
      data: data
    }]
  }
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.3/Chart.min.js"></script>
<canvas id="chart" height="75"></canvas>

(Note: the use of Math.round() could cause certain number sets not to equal 100% when summed.)

score:1

If you already have an array of legends, you can add the percentage to it with this code. Here i is the index of the current element in the array.

data=[2,4,5,7];
legend=['A','B','C','D']
total = data.reduce((accumulator, currentValue) => parseInt(accumulator) + parseInt(currentValue));
labelsvalues = data.map(function(value,i){
let p= Math.round((value / total) * 100) + '%';
return legend[i]+' '+p;
});

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