请问为什么设置grid.tooltip 却没有效果？echarts 折线配置项内容和展示

      option = {
    
    grid: [
        {
            height: "50%", 
            tooltip: {
                show: true,
                trigger: 'axis',
                axisPointer: {
                    type: 'cross'
                }
            }
            
        }, 
        {height: "40%", top: "65%"}
    ],
    xAxis: [{
        type: 'category',
        data: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
    },{
        type: 'category',
        gridIndex: 1,
        data: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
    },],
    yAxis: [{
        type: 'value'
    },{
        type: 'value',
        gridIndex: 1,
    },],
    series: [{
        data: [820, 932, 901, 934, 1290, 1330, 1320],
        type: 'line',
        smooth: true
    },
    {
        data: [920, 932, 901, 934, 1290, 1330, 1320],
        xAxisIndex: 1,
        yAxisIndex: 1,
        type: 'line',
        smooth: true
    }],
   
};