echarts，xAxis类型为time时，boundaryGap坐标轴两边留白策略设置无效。

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%']
					},
			"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。