ChartJS: datalabels: show percentage value in Pie piece

Question

I have a piechart with four labels:

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

Using chartjs-plugin-datalabels plugin I wanted to show percentage value in each Pie piece with below code:

formatter: (value, ctx) => {

        let datasets = ctx.chart.data.datasets;

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

I am getting 100% value for all the pie pieces, instead of respective percentages. Here is the JSFiddle (https://jsfiddle.net/kingBethal/a1Lvn4eb/7/)

Answer 1

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
});

Answer 2

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>