在chartjs中，在顶部和底部的边缘对半切割的点。

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: 'Dataset',
            data: [10, 20, 30, 40, 50, 60, 70],
            borderColor: 'rgba(255, 99, 132, 1)',
            borderWidth: 1
        }]
    },
    options: {
        plugins: {
            afterDraw: function(chart) {
                var ctx = chart.ctx;
                var xAxis = chart.scales['x-axis-0'];
                var yAxis = chart.scales['y-axis-0'];
                var dataset = chart.data.datasets[0];
                var data = dataset.data;
                
                for (var i = 0; i < data.length; i++) {
                    var x = xAxis.getPixelForValue(i);
                    var y = yAxis.getPixelForValue(data[i]);
                    
                    ctx.beginPath();
                    ctx.arc(x, y, 5, 0, 2 * Math.PI);
                    ctx.closePath();
                    ctx.fillStyle = 'rgba(255, 99, 132, 1)';
                    ctx.fill();
                }
            }
        }
    }
});