Pie Chart Legend - Chart.js

Following on from tektiv's answer, I've modified it for ES6 which my linter requires;

options: {
  legend: {
    labels: {
      generateLabels: (chart) => {
        const { data } = chart;
        if (data.labels.length && data.datasets.length) {
          return data.labels.map((label, i) => {
            const meta = chart.getDatasetMeta(0);
            const ds = data.datasets[0];
            const arc = meta.data[i];
            const custom = (arc && arc.custom) || {};
            const { getValueAtIndexOrDefault } = Chart.helpers;
            const arcOpts = chart.options.elements.arc;
            const fill = custom.backgroundColor ? custom.backgroundColor : getValueAtIndexOrDefault(ds.backgroundColor, i, arcOpts.backgroundColor);
            const stroke = custom.borderColor ? custom.borderColor : getValueAtIndexOrDefault(ds.borderColor, i, arcOpts.borderColor);
            const bw = custom.borderWidth ? custom.borderWidth : getValueAtIndexOrDefault(ds.borderWidth, i, arcOpts.borderWidth);
            const value = chart.config.data.datasets[arc._datasetIndex].data[arc._index];

            return {
              text: `${label}: ${value}`,
              fillStyle: fill,
              strokeStyle: stroke,
              lineWidth: bw,
              hidden: Number.isNaN(ds.data[i]) || meta.data[i].hidden,
              index: i,
            };
          });
        }
        return [];
      },
    },
  },
},

You need to edit the generateLabels property in your options :

options: {
    legend: {
        labels: {
            generateLabels: function(chart) {
                 // Here
            }
        }
    }
}

---

Since it is quite a mess to create on your own a great template. I suggest using the same function as in the source code and then edit what is needed.

Here are a small jsFiddle, where you can see how it works (edited lines - from 38 - are commented), and its result :

Maybe this is a hacky solution, but for me seems simpler.

The filter parameter

ChartJS legend options have a filter parameter. This is a function that is called for each legend item, and that returns true/false whether you want to show this item in the legend or not.

filter has 2 arguments:

• legendItem : The legend item to show/omit. Its properties are described here
• data : The data object passed to the chart.

The hack

Since JS passes objects by reference, and filter is called for each legend item, then you can mutate the legendItem object to show the text that you want.

legend : {
  labels: {
    filter: (legendItem, data) => {
      // First, retrieve the data corresponding to that label
      const label = legendItem.text
      const labelIndex = _.findIndex(data.labels, (labelName) => labelName === label) // I'm using lodash here
      const qtd = data.datasets[0].data[labelIndex]
      
      // Second, mutate the legendItem to include the new text
      legendItem.text = `${legendItem.text} : ${qtd}`
      
      // Third, the filter method expects a bool, so return true to show the modified legendItem in the legend
      return true
    }
  }
}