ChartJS自定义工具提示不会在标签上呈现背景(仅显示标题)

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

var ctx = document.getElementById('myChart').getContext('2d');
var myChart = new Chart(ctx, {
    type: 'bar',
    data: {
        labels: ['Red', 'Blue', 'Yellow', 'Green', 'Purple', 'Orange'],
        datasets: [{
            label: 'My Dataset',
            data: [12, 19, 3, 5, 2, 3],
            backgroundColor: 'rgba(255, 99, 132, 0.2)',
            borderColor: 'rgba(255, 99, 132, 1)',
            borderWidth: 1
        }]
    },
    options: {
        tooltips: {
            enabled: true,
            mode: 'single',
            backgroundColor: 'rgba(0, 0, 0, 0.8)',
            titleFontColor: '#fff',
            bodyFontColor: '#fff',
            callbacks: {
                title: function(tooltipItems, data) {
                    return data.labels[tooltipItems[0].index];
                },
                label: function(tooltipItem, data) {
                    return 'Value: ' + tooltipItem.yLabel;
                }
            }
        }
    }
});