读者提问：如何实现多层级轴标签

srcData = [
    {group: 'Letters', subGroup: 'lowercase', name: 'a', value: 11},
    {group: 'Letters', subGroup: 'lowercase', name: 'b', value: 41},
    {group: 'Letters', subGroup: 'uppercase', name: 'C', value: 16},
    {group: 'Letters', subGroup: 'uppercase', name: 'D', value: 32},
    {group: 'Letters', subGroup: 'uppercase', name: 'E', value: 34},
    {group: 'Numbers', subGroup: 'Integer', name: 1, value: 12},
    {group: 'Numbers', subGroup: 'Integer', name: 2, value: 35},
    {group: 'Numbers', subGroup: 'Integer', name: 3, value: 23},
    {group: 'Numbers', subGroup: 'Decimal', name: 2.3, value: 16},
    {group: 'Numbers', subGroup: 'Decimal', name: 4.5, value: 12}
];

covertData = function(src) {
    var nameList = [];  // X 轴数据
    var valueList = []; // Y 轴数据

    var groupLocationList = []; // 分组位置数据，存入[1.5, 5]这种
    var subGroupLocationList = []; // 二级分组位置列表

    var groupTickList = [0]; // 分组位置刻度位置数据，整数列表
    var subGroupTickList = [0]; // 二级分组刻度位置数据

    // 用于记录上次遍历到的分组/二级分组名称，以及该名称出现过的次数
    var groupTmp = {
        name: '',
        count: 0
    };
    var subGroupTmp = {
        name: '',
        count: 0
    };

    // 遍历源数据，生成所需的图表数据、分组轴标签、分组轴刻度数据
    for (var i = 0; i < src.length; i++) {

        nameList.push(src[i].name); 
        valueList.push(src[i].value);

        // 当一个分组/二次分组名称遍历完成时，计算出该分组的标签、刻度位置
        // 然后重新开始计数，这里分别用了两种写法，三元表达式和 if-else 语句
        i === src.length - 1 || groupTmp.name !== '' && groupTmp.name !== src[i + 1].group ?
            (groupLocationList.push(i - groupTmp.count / 2),
                groupTmp.name = '',
                groupTmp.count = 0,
                groupTickList.push(i + 1)) :
            (groupTmp.name = src[i].group,
                groupTmp.count++);

        if (i === src.length - 1 || subGroupTmp.name !== '' && subGroupTmp.name !== src[i + 1].subGroup) {
            subGroupLocationList.push(i - subGroupTmp.count / 2);
            subGroupTmp.name = '';
            subGroupTmp.count = 0;
            subGroupTickList.push(i + 1);
        } else {
            subGroupTmp.name = src[i].subGroup;
            subGroupTmp.count++;
        }

    }

    return {
        nameList: nameList,
        valueList: valueList,
        groupLocationList: groupLocationList,
        subGroupLocationList: subGroupLocationList,
        groupTickList: groupTickList,
        subGroupTickList: subGroupTickList
    };
};

dstData = covertData(srcData);
console.log(dstData);

option = {
    // 准备 3 个同一位置叠放的 grid （直角坐标系）
    grid: [{
        bottom: '20%'
    }, {
        bottom: '20%'
    }, {
        bottom: '20%'
    }],
    
    // 准备 3 个 X 轴，第一个放数据，后两个放分组标签、刻度
    xAxis: [{
        gridIndex: 0,
        type: 'category',
        data: dstData.nameList,
        axisTick: {
            length: 20
        }
    }, {
        gridIndex: 1,
        type: 'category',
        data: dstData.nameList,
        axisLabel: {
            margin: 100,
            formatter: function(value, index) {
                // 使用回调函数定义分组标签显示的内容及位置
                // 当位置为整数时放中间，在两个整数之间时加点空格
                // 逻辑控制使用三元表达式嵌套
                var retVal = dstData.groupLocationList.indexOf(index) != -1 ?
                    srcData[index].group :
                    dstData.groupLocationList.indexOf(index + 0.5) != -1 ?
                    '  ' + srcData[index].group :
                    '';
                    
                return retVal;
            }
        },
        axisTick: {
            length: 100, // 刻度长度
            // 用回调函数定义刻度的显示/隐藏
            interval: (index) => dstData.groupTickList.indexOf(index) != -1
        }
    }, {
        gridIndex: 2,
        type: 'category',
        data: dstData.nameList,
        axisLabel: {
            margin: 50,
            formatter: function(value, index) {
                // 使用回调函数定义二级分组标签显示的内容及位置
                // 当位置为整数时放中间，在两个整数之间时加点空格
                // 逻辑控制用的 if 语句结合 return
                if (dstData.subGroupLocationList.indexOf(index) != -1) {
                    return srcData[index].subGroup;
                }
                
                if (dstData.subGroupLocationList.indexOf(index + 0.5) != -1) {
                    return '  ' + srcData[index].subGroup;
                }
            }
        },
        axisTick: {
            length: 50,
            interval: (index) => dstData.subGroupTickList.indexOf(index) != -1
        }
    }],
    yAxis: [{
        gridIndex: 0,
    }, {
        gridIndex: 1,
    }, {
        gridIndex: 2,
    }],

    series: [{
        type: 'bar',
        name: 'test',
        label: {
            show: true,
            position: 'top'
        },
        data: dstData.valueList
    }]
};