ChartJS: datalabels: show percentage value in Pie piece

I like to add a little in accepted answer, ctx.chart.data.datasets[0].data always gives you entire data even if you filter out some data by clicking on legend, means you will always get same percentage for a country even if you filter out some countries.

I have used context.dataset._meta[0].total to get the filtered total.

Here is the working snippet:

var data = [{
  data: [50, 55, 60, 33],
  backgroundColor: [
    "#4b77a9",
    "#5f255f",
    "#d21243",
    "#B27200"
  ],
  borderColor: "#fff"
}];

var options = {
  tooltips: {
    enabled: true
  },
  plugins: {
    datalabels: {
      formatter: (value, ctx) => {

        let sum = ctx.dataset._meta[0].total;
        let percentage = (value * 100 / sum).toFixed(2) + "%";
        return percentage;


      },
      color: '#fff',
    }
  }
};


var ctx = document.getElementById("pie-chart").getContext('2d');
var myChart = new Chart(ctx, {
  type: 'pie',
  data: {
  labels: ['India', 'China', 'US', 'Canada'],
    datasets: data
  },
  options: options
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.7.2/Chart.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/[email protected]"></script>
<canvas id="pie-chart"></canvas>

This is update for chart.js >= 3.x and chartjs-plugin-datalabels >= 2.x

chartjs-plugin-datalabels plugin no longer registers itself automatically (docs)

You have to do it manually

Register plugin for all charts

Chart.register(ChartDataLabels)

or only to specific charts

var chart = new Chart(ctx, {
  plugins: [ChartDataLabels],
  options: {
    // ...
  }
})

And here's the code below to render the pie chart

var data = [{
  data: [50, 55, 60, 33],
  backgroundColor: [
    "#4b77a9",
    "#5f255f",
    "#d21243",
    "#B27200"
  ],
  borderColor: "#fff"
}];

var options = {
  tooltips: {
    enabled: false
  },
  plugins: {
    datalabels: {
      formatter: (value, ctx) => {
        const datapoints = ctx.chart.data.datasets[0].data
         const total = datapoints.reduce((total, datapoint) => total + datapoint, 0)
        const percentage = value / total * 100
        return percentage.toFixed(2) + "%";
      },
      color: '#fff',
    }
  }
};

var ctx = document.getElementById("pie-chart").getContext('2d');
var myChart = new Chart(ctx, {
  type: 'pie',
  data: {
  labels: ['India', 'China', 'US', 'Canada'],
    datasets: data
  },
  options: options,
  plugins: [ChartDataLabels],
});

  <script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/3.6.2/chart.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/chartjs-plugin-datalabels/2.0.0/chartjs-plugin-datalabels.min.js"></script>
  <style>
.container {
width: 40%;
margin: 5px auto;
}
  </style>
  <div class="container">
<canvas id="pie-chart"></canvas>
  </div>

The problem is how you're calculating sum. See below.

Fiddle

 var data = [{
   data: [50, 55, 60, 33],
   labels: ["India", "China", "US", "Canada"],
   backgroundColor: [
     "#4b77a9",
     "#5f255f",
     "#d21243",
     "#B27200"
   ],
   borderColor: "#fff"
 }];

 var options = {
   tooltips: {
     enabled: false
   },
   plugins: {
     datalabels: {
       formatter: (value, ctx) => {

         let datasets = ctx.chart.data.datasets;

         if (datasets.indexOf(ctx.dataset) === datasets.length - 1) {
           let sum = datasets[0].data.reduce((a, b) => a + b, 0);
           let percentage = Math.round((value / sum) * 100) + '%';
           return percentage;
         } else {
           return percentage;
         }
       },
       color: '#fff',
     }
   }
 };


 var ctx = document.getElementById("pie-chart").getContext('2d');
 var myChart = new Chart(ctx, {
   type: 'pie',
   data: {
     datasets: data
   },
   options: options
 });

Updated fiddle with 2 decimal precision.

You were not computing the sum, instead storing the current value in sum only for every value.

Here is the working fiddle : https://jsfiddle.net/a1Lvn4eb/55/

var data = [{
    data: [50, 55, 60, 33],
    labels: ["India", "China", "US", "Canada"],
    backgroundColor: [
        "#4b77a9",
        "#5f255f",
        "#d21243",
        "#B27200"
    ],
    borderColor: "#fff"
}];

var options = {
    tooltips: {
        enabled: false
    },
    plugins: {
        datalabels: {
            formatter: (value, ctx) => {
                let sum = 0;
                let dataArr = ctx.chart.data.datasets[0].data;
                dataArr.map(data => {
                    sum += data;
                });
                let percentage = (value*100 / sum).toFixed(2)+"%";
                return percentage;
            },
            color: '#fff',
        }
    }
};

var ctx = document.getElementById("pie-chart").getContext('2d');
var myChart = new Chart(ctx, {
    type: 'pie',
    data: {
        datasets: data
    },
    options: options
});