在Highcharts上堆叠y轴，如何使其成为一个？

Highcharts.chart('container', {
    chart: {
        type: 'column'
    },
    title: {
        text: '堆叠y轴示例'
    },
    xAxis: {
        categories: ['A', 'B', 'C', 'D', 'E']
    },
    yAxis: {
        min: 0,
        title: {
            text: '数值'
        },
        stackLabels: {
            enabled: true,
            style: {
                fontWeight: 'bold',
                color: 'gray'
            }
        }
    },
    legend: {
        align: 'right',
        verticalAlign: 'top',
        layout: 'vertical',
        x: 0,
        y: 100
    },
    plotOptions: {
        column: {
            stacking: 'normal' // 设置堆叠方式为normal
        }
    },
    series: [{
        name: '系列1',
        data: [5, 3, 4, 7, 2]
    }, {
        name: '系列2',
        data: [2, 2, 3, 2, 1]
    }, {
        name: '系列3',
        data: [3, 4, 4, 2, 5]
    }]
});