chartjs -如果值低于之前的值，如何使用不同的颜色？

var ctx = document.getElementById('myChart').getContext('2d');
var myChart = new Chart(ctx, {
    type: 'line',
    data: {
        labels: ['January', 'February', 'March', 'April', 'May', 'June', 'July'],
        datasets: [{
            label: 'My Dataset',
            data: [10, 20, 15, 30, 25, 5, 40],
            fill: false,
            borderColor: '#007bff',
            tension: 0.4,
        }]
    },
    options: {
        // 配置其他参数
    }
});

myChart.options.plugins = {
    afterUpdate: function(chart) {
        var datasets = chart.data.datasets;
        var ctx = chart.ctx;

        datasets.forEach(function(dataset) {
            var meta = chart.getDatasetMeta(dataset.label);
            var data = dataset.data;

            meta.data.forEach(function(point, index) {
                if (index > 0 && data[index] < data[index - 1]) {
                    point.custom = point.custom || {};
                    point.custom.backgroundColor = '#dc3545';
                } else {
                    point.custom = point.custom || {};
                    point.custom.backgroundColor = '#007bff';
                }
            });
        });

        chart.update();
    }
};

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