echart pie chart custom percentage

tooltip : {
    trigger: 'item',
    formatter: function(a){
        return (a['name']+'<br/>'+a['value'] +':'+a['data'].datas[0]); //custom percentage
    }
},
calculable: false,
series: [
    {
        name: 'Access source',
        type: 'pie',
        radius: '55%',
        center: ['50%', '60%'],
        data: [
            {value: 335, name: 'Direct access', datas: ['11.20%']},
            {value: 310, name: 'email marketing', datas: ['31%']},
            {value: 234, name: 'Affiliate Ads', datas: ['24%']},
            {value: 135, name: 'video ad', datas: ['16%']},
            {value: 1548, name: 'Search Engine', datas: ['13.31%']},
        ]
    }
]