echarts，xAxis类型为time时，boundaryGap设置无效。

Question

echarts官网的api中有这么一段：
xAxis.boundaryGap
坐标轴两边留白策略，类目轴和非类目轴的设置和表现不一样。
类目轴中 boundaryGap 可以配置为 true 和 false。默认为 true，这时候刻度只是作为分隔线，标签和数据点都会在两个刻度之间的带(band)中间。
非类目轴，包括时间，数值，对数轴，boundaryGap是一个两个值的数组，分别表示数据最小值和最大值的延伸范围，可以直接设置数值或者相对的百分比，在设置 min 和 max 后无效。 示例：
boundaryGap: ['20%', '20%']

可是为什么x轴的type:time类型时候就无效呢？？？
源码如下：

option = {
      "title":{"text":"玩家充值","link":"javascript: void(0)","target":"self","subtext":"单位：元","x":"left"},
             toolbox: {
                show: true,
                orient:'vertical',
                right:'25',
                feature: {
                    magicType: {
                         type: ['line', 'bar', 'stack', 'tiled']    //折线与柱形转化
                        
                    },
                    //dataView: {},                //数据视图
                    restore: {},                //还原
                    saveAsImage: {}                //保存为图片
                }
            },
            "tooltip":{"trigger":"axis","axisPointer":{"type":"line"},formatter:''},
            "legend":{
                "left":"center",
                "data":["捕鱼","斗地主","牛牛"] 
                },
            "grid":{"left":"3%","right":"4%","bottom":"3%","containLabel":true},
            "xAxis":{"type":"time",
                     "boundaryGap":['20%', '20%']    **//这里无效！！！**
                    },
            "yAxis":{"type":"value"},
            "series":[
                {"name":"斗地主","type":"bar","stack":"总量","data":[["2017-12-04 00:00:00",130.888],["2017-12-05 00:00:00",380.3698],["2017-12-06 00:00:00",320],["2017-12-07 00:00:00",320]]},
                {"name":"捕鱼","type":"bar","stack":"总量","data":[["2017-12-04 00:00:00",125],["2017-12-05 00:00:00",410],["2017-12-06 00:00:00",520],["2017-12-07 00:00:00",320]]},
                {"name":"牛牛","type":"bar","stack":"总量","data":[["2017-12-04 00:00:00",275],["2017-12-05 00:00:00",220],["2017-12-06 00:00:00",498],["2017-12-07 00:00:00",320]]}]
};

显示结果如下：

两边并没有留白？
求大佬赐教，跪谢。