legend滚动条问题echarts 饼配置项内容和展示

      app.title = '嵌套环形图';
/*
借这个饼图用一下，问一个问题，
图例比这个还多的情况下，
不管怎么空出空地都会压到饼图，很不美观
我想把legend放在一个div当中然后做一个滚动条 
当用户需要的时候他自己去下拉选择，请问怎么实现
原谅小白的JS功底不够吧

*/
option = {
    tooltip: {
        trigger: 'item',
        formatter: "{a} <br/>{b}: {c} ({d}%)"
    },
    legend: {
        orient: 'vertical',
        x: 'left',
        data:['直达','营销广告','搜索引擎','邮件营销','联盟广告','视频广告','百度','谷歌','必应','其他']
    },
    series: [
        {
            name:'访问来源',
            type:'pie',
            selectedMode: 'single',
            radius: [0, '30%'],

            label: {
                normal: {
                    position: 'inner'
                }
            },
            labelLine: {
                normal: {
                    show: false
                }
            },
            data:[
                {value:335, name:'直达', selected:true},
                {value:679, name:'营销广告'},
                {value:1548, name:'搜索引擎'}
            ]
        },
        {
            name:'访问来源',
            type:'pie',
            radius: ['40%', '55%'],

            data:[
                {value:335, name:'直达'},
                {value:310, name:'邮件营销'},
                {value:234, name:'联盟广告'},
                {value:135, name:'视频广告'},
                {value:1048, name:'百度'},
                {value:251, name:'谷歌'},
                {value:147, name:'必应'},
                {value:102, name:'其他'}
            ]
        }
    ]
};