如何使用Chartjs对图例条目进行分组？

<canvas id="myChart"></canvas>

var ctx = document.getElementById('myChart').getContext('2d');
var myChart = new Chart(ctx, {
    type: 'bar',
    data: {
        labels: ['Group A', 'Group B', 'Group C', 'Group D'],
        datasets: [{
            label: 'Data 1',
            data: [12, 19, 3, 5],
            backgroundColor: 'rgba(255, 99, 132, 0.2)'
        }, {
            label: 'Data 2',
            data: [7, 11, 5, 8],
            backgroundColor: 'rgba(54, 162, 235, 0.2)'
        }]
    },
    options: {
        legend: {
            display: true,
            labels: {
                generateLabels: function(chart) {
                    var data = chart.data;
                    if (data.labels.length && data.datasets.length) {
                        return data.labels.map(function(label, i) {
                            var meta = chart.getDatasetMeta(0);
                            var ds = data.datasets[0];
                            var arc = meta.data[i];
                            var custom = arc && arc.custom || {};
                            var getValueAtIndexOrDefault = Chart.helpers.getValueAtIndexOrDefault;
                            var arcOpts = chart.options.elements.arc;
                            var fill = custom.backgroundColor ? custom.backgroundColor : getValueAtIndexOrDefault(ds.backgroundColor, i, arcOpts.backgroundColor);
                            var stroke = custom.borderColor ? custom.borderColor : getValueAtIndexOrDefault(ds.borderColor, i, arcOpts.borderColor);
                            var bw = custom.borderWidth ? custom.borderWidth : getValueAtIndexOrDefault(ds.borderWidth, i, arcOpts.borderWidth);

                            // 分组逻辑
                            if (i < 2) {
                                return {
                                    text: label,
                                    fillStyle: fill,
                                    strokeStyle: stroke,
                                    lineWidth: bw,
                                    hidden: isNaN(ds.data[i]) || meta.data[i].hidden,

                                    // Extra data used for toggling the correct item
                                    index: i
                                };
                            } else {
                                return {
                                    text: label,
                                    fillStyle: fill,
                                    strokeStyle: stroke,
                                    lineWidth: bw,
                                    hidden: isNaN(ds.data[i]) || meta.data[i].hidden,

                                    // Extra data used for toggling the correct item
                                    index: i,
                                    // 分组逻辑
                                    group: 'Group 2'
                                };
                            }
                        });
                    }
                    return [];
                }
            }
        }
    }
});