Chart.js – how to add icon (or custom html-code) to legend item?

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Summary

The problem at hand is customizing the legend of a Chart.js doughnut chart to display a country icon before the country name. The user has tried using the generateLabels callback, but it displays HTML code as text instead of rendering the icon.

Root Cause

The root cause of this issue is that the generateLabels callback returns an object with a text property, which is treated as plain text by Chart.js. To render HTML content, we need to use a different approach.

  • The generateLabels callback is used to customize the legend labels.
  • The text property of the returned object is treated as plain text.
  • HTML code is not rendered, but instead displayed as text.

Why This Happens in Real Systems

This issue occurs in real systems because Chart.js does not support rendering HTML content in the legend labels by default. The generateLabels callback is designed to return plain text, and any HTML code is escaped and displayed as text.

  • Chart.js is designed to work with plain text labels.
  • HTML content is not supported in legend labels by default.
  • The generateLabels callback is not intended to return HTML content.

Real-World Impact

The real-world impact of this issue is that users may not be able to customize the legend of their charts as desired. This can lead to a poor user experience, especially if the legend is a critical part of the chart.

  • Users may not be able to customize the legend as desired.
  • The chart may not be as effective in communicating the desired information.
  • The user experience may be negatively impacted.

Example or Code

const countriesChartConfig = {
  type: 'doughnut',
  data: countriesData,
  options: {
    plugins: {
      legend: {
        position: 'right',
        labels: {
          generateLabels: function(chart) {
            return chart.data.labels.map(function(label, i) {
              let backgroundColor = chart.data.datasets[0].backgroundColor[i];
              let count = chart.data.datasets[0].data[i];
              let total = 0;
              let dataArr = chart.data.datasets[0].data;
              dataArr.map(data => {
                total += data;
              });
              let percent = (count / total) * 100;
              country_code = chart.data.datasets[0].data[i].country_code;
              return {
                text: flag(country_code) + ' ' + label + ' - ' + count + ' (' + percent.toFixed(2) + '%)',
                fillStyle: backgroundColor,
                // Add this to render the icon
                html: true
              };
            });
          }
        }
      }
    },
    animation: false,
    maintainAspectRatio: false,
    parsing: {
      key: 'count'
    }
  }
};

How Senior Engineers Fix It

Senior engineers fix this issue by using a custom legend implementation that renders HTML content. This can be achieved by using the legend plugin and setting the html property to true in the generateLabels callback.

  • Use a custom legend implementation.
  • Set the html property to true in the generateLabels callback.
  • Render the icon using HTML content.

Why Juniors Miss It

Juniors may miss this issue because they may not be familiar with the limitations of Chart.js. They may also not understand how to customize the legend using the generateLabels callback.

  • Juniors may not be familiar with the limitations of Chart.js.
  • They may not understand how to customize the legend using the generateLabels callback.
  • They may not know how to render HTML content in the legend labels.