How can I show a subset of data on pie pieces in Chart.JS while still displaying the superset when hovering?

Question

I\'ve got a pie chart that looks like this when hovering over a piece of pie:

Except for wanting the legend to the right instead of on top, I\'m fairly cont

Answer 1

The following solution is using the same calculation as lamelemon's but using Chart.js plugins, which brings additional benefits :

• There is no longer this blink effect caused by the animation.onComplete since it occurs immediatly without waiting for antything.

• The tooltip is above the text, which is more natural.

Follows the plugin that does it :

Chart.pluginService.register({
    afterDatasetsDraw: function(chartInstance) {

        // We get the canvas context
        var ctx = chartInstance.chart.ctx;

        // And set the properties we need
        ctx.font = Chart.helpers.fontString(14, 'bold', Chart.defaults.global.defaultFontFamily);
        ctx.textAlign = 'center';
        ctx.textBaseline = 'bottom';
        ctx.fillStyle = '#666';

        // For ervery dataset ...
        chartInstance.config.data.datasets.forEach(function(dataset) {

            // For every data in the dataset ...
            for (var i = 0; i < dataset.data.length; i++) {

                // We get all the properties & values we need
                var model = dataset._meta[Object.keys(dataset._meta)[0]].data[i]._model,
                    total = dataset._meta[Object.keys(dataset._meta)[0]].total,
                    mid_radius = model.innerRadius + (model.outerRadius - model.innerRadius) / 2,
                    start_angle = model.startAngle,
                    end_angle = model.endAngle,
                    mid_angle = start_angle + (end_angle - start_angle) / 2;

                // We calculate the right positions for our text
                var x = mid_radius * 1.5 * Math.cos(mid_angle);
                var y = mid_radius * 1.5 * Math.sin(mid_angle);

                // We calculate the percentage
                var percent = String(Math.round(dataset.data[i] / total * 100)) + "%";
                // And display it
                ctx.fillText(percent, model.x + x, model.y + y);
            }
        });
    }
});

---

You can check this plugin working on this jsFiddle, and here is the result :

Answer 2

Alright looks like you have some simple syntax errors in your updated code that probably happened with copying over. Ignoring those, you can manage this by creating a function in your animation options. Here is the fiddle. Thanks to Hung Tran's work here.

animation: {
  duration: 0,
  easing: "easeOutQuart",
  onComplete: function () {
    var ctx = this.chart.ctx;
    ctx.font = Chart.helpers.fontString(Chart.defaults.global.defaultFontFamily, 'normal', Chart.defaults.global.defaultFontFamily);
    ctx.textAlign = 'center';
    ctx.textBaseline = 'bottom';

    this.data.datasets.forEach(function (dataset) {

      for (var i = 0; i < dataset.data.length; i++) {
        var model = dataset._meta[Object.keys(dataset._meta)[0]].data[i]._model,
            total = dataset._meta[Object.keys(dataset._meta)[0]].total,
            mid_radius = model.innerRadius + (model.outerRadius - model.innerRadius)/2,
            start_angle = model.startAngle,
            end_angle = model.endAngle,
            mid_angle = start_angle + (end_angle - start_angle)/2;

        var x = mid_radius * Math.cos(mid_angle);
        var y = mid_radius * Math.sin(mid_angle);

        ctx.fillStyle = '#fff';
        if (i == 3){ // Darker text color for lighter background
          ctx.fillStyle = '#444';
        }
        var percent = String(Math.round(dataset.data[i]/total*100)) + "%";
        // this prints the data number
        ctx.fillText(dataset.data[i], model.x + x, model.y + y);
        // this prints the percentage
        ctx.fillText(percent, model.x + x, model.y + y + 15);
      }
    });               
  }
}