怎么去除折线左右两边的线echarts 配置项内容和展示

要怎么去掉图中最左和最右的两条线

      option = {
    legend: {
                 y :'bottom',
                 data: ["通知公告", "我的留言","在线服务"]
             },

             tooltip: {
                formatter: "{b}: <br/>{a}: {c}",
                trigger: "item"
            },
             xAxis: [
                 {
                    axisLine: {
                        show:false
                    },
                    splitLine: {
                        show: false
                    },
                     data : ['2017/5/12','2017/5/12','2017/5/12','2017/5/12','2017/5/12','2017/5/12','2017/5/12']
                 }
             ],
             color : ["#9BBB59","#C0504D","#4F81BD"],
             yAxis: [
                 {
                    axisLine: {
                        show:false
                    },
                     type: "log"
                 }
             ],
             series: [
                 {
                     name: "通知公告",
                     type: "line",
                     data:[180, 200, 220, 280, 180, 320, 400]

                 },
                 {
                     name: "我的留言",
                     type: "line"

                 },
                 {
                     name: "在线服务",
                     type: "line"
                 }
             ]
};