echarts3如何清空上一次加载的series数据

getEcharts() {
      let _bar = this.$refs.bar,
      this.http.post('xx/xx/xx', {
        id: JSON.stringify(this.selectParkIds[0]),
        fixedDate: JSON.stringify(this.today)
      }).then(data => {
        _bar.clear()
       if (data) return
        if (data.resultData) {
           _bar.mergeOptions(getBarOption(inParkDataList)
        }
      })
    },

const getBarOption = (data) => {
    let xAxisAry = [], //x轴数据
        legend = [],
        inParkExitsFist = [], 
        inParkExitsList = [], 
        dataAllAry = [],
        alignCenter = ''
    for (let obj of data) {
        xAxisAry.push(obj.hour)
        inParkExitsList.push(obj.inParkExits)
    }
    inParkExitsFirst = data[0].inParkExits
    for (let obj of inParkExitsFist) {
        legend.push(obj.parkExitName)
    }
    legend.length > 10 ? alignCenter = '12%' : alignCenter = '30%'
       
    for (let lenObj of legend) {
        let dataAry = [],
            dataObj = {}
        for (let ary of inParkExitsList) {
            ary.forEach(function(element) {
                if (element.parkExitName == lenObj) {
                    dataAry.push(element.inParkCount)
                }
            });
        }
        dataObj.name = lenObj
        dataObj.data = dataAry
        dataObj.type = "bar"
        dataAllAry.push(dataObj)
    }
    return {
        tooltip: {
            trigger: 'axis',
            axisPointer: { // 坐标轴指示器，坐标轴触发有效
                type: 'shadow' // 默认为直线，可选为：'line' | 'shadow'
            }
        },
        title: {
            //text: obj.title
        },
        xAxis: {
            data: xAxisAry
        },
        yAxis: {
            axisLabel: { show: true },
            name: "单位(辆)"
        },
        legend: {
            data: legend,
            left: alignCenter,
            y: "bottom",
            itemGap: 30,
            align: 'auto',
        },
        grid: {
            bottom: '30%'
        },
        series: dataAllAry,
        
        color: [
            '#3b4ca9', '#1689ce', '#1fc659', '#f98b24', '#e2346e', '#d52f30',
            '#5d6dbe', '#1a9ce2', '#25e47b', '#fda639', '#f44c86', '#eb393a',
            '#5f77b1', '#34b6f3', '#6cf090', '#fdad2a', '#f06997', '#ec5454',
            '#7a88c9', '#59c7ef', '#6feeaf', '#feb657', '#f290b1', '#e27375',
            '#9fa9d8', '#84d5f8', '#bbf5cb', '#fecc86', '#f6bbd0', '#ed9a9b'
        ],
    }
}