關(guān)于ECharts折線圖值的顯示問題

我這邊有兩條折線，發(fā)現(xiàn)最上面那條折線圖的值沒有真正顯示在y軸上，比如最上面的第一個點，值是30，為什么點要在y軸50上呢？我想顯示在y軸30上，該怎么弄呢？

option = {
        title: {
            text: '折線圖堆疊'
        },
        tooltip: {
            trigger: 'axis'
        },
        legend: {
            data:['張三','李四']
        },
        grid: {
            left: '3%',
            right: '4%',
            bottom: '3%',
            containLabel: true
        },
        toolbox: {
            feature: {
                saveAsImage: {}
            }
        },
        xAxis: {
            type: 'category',
            boundaryGap: false,
            data: ['1','2','3','4','5','6','7','8','9']
        },
        yAxis: {
            type: 'value'
        },
        series: [
            {
                name:'張三',
                type:'line',
                stack: '總量',
                data:[20, 17.6, 12.5, 9.7, 7.9, 6.7, 5.8, 5.1, 4.6]
            },
            {
                name:'李四',
                type:'line',
                stack: '總量',
                data:[30, 14.38, 4.79, 13.01, 15.07, 13.7, 6.85, 2.05, 4.79]
            }
        ]
    };