改变Y轴悬浮显示数值

描述:当前是关于Echarts图表中的 折线图 示例。
 
            option={
    tooltip: {
        trigger: 'axis',
         formatter: function (params){
             
             var res='';
             for (var i = 0; i < params.length; i++) {
                 res+=  params[i].seriesName + ' : ' + params[i].value+'%' + '<br/>';
             }
             return res;
		       },
        axisPointer: {
            type: 'cross',
            label: {
                backgroundColor: '#6a7985'
            }
        }
    },
    legend: {
        data: [
            '邮件营销',
            '联盟广告',
            '视频广告',
        ]
    },
    toolbox: {
        feature: {
            saveAsImage: {
                
            }
        }
    },
    grid: {
        left: '3%',
        right: '4%',
        bottom: '3%',
        containLabel: true
    },
    xAxis: [
        {
            type: 'category',
            boundaryGap: false,
            data: [
                '周一',
                '周二',
                '周三',
                '周四',
                '周五',
                '周六',
                '周日'
            ]
        }
    ],
    yAxis: [
        
        {
            
            axisLabel:{
                formatter :function(a){
                    return a+"%";
                }
            }
        },
        {
            type : 'value'
        }
    ],
    series: [
        {
            name: '邮件营销',
            type: 'line',
            stack: '总量',
            areaStyle: {
                normal: {
                    color: '#bce0eb'
                }
            },
            itemStyle: {
                normal: {
                    color: '#bce0eb'
                }
            },
            data: [
                12.0,
                13.2,
                10.1,
                13.4,
                9.0,
                23.0,
                21.0
            ]
        },
        {
            name: '联盟广告',
            type: 'line',
            stack: '总量',
            areaStyle: {
                normal: {
                    color: '#00a5d5'
                }
            },
            itemStyle: {
                normal: {
                    color: '#00a5d5'
                }
            },
            data: [
                22.0,
                18.2,
                19.1,
                23.4,
                29.0,
                33.0,
                31.0
            ]
        },
        {
            name: '视频广告',
            type: 'line',
            stack: '总量',
            areaStyle: {
                normal: {
                    color: '#006c8c'
                }
            },
            itemStyle: {
                normal: {
                    color: '#006c8c'
                }
            },
            data: [
                23.0,
                12.2,
                13.1,
                18.4,
                23.0,
                25.0,
                33.0
            ]
        }
    ]
};