问题请教：关于Xbar-R控制图中遇到的问题

背景：我收集了130个数据来做Xbar-R控制图，每5个数据为一组，其中有一组数据的有个数据丢失，只记录了4个。结果出现下图所示的情形，但不知道为什么会出现此情形，请朋友们指点。谢谢。

数据如下：
Date Data
12/08/05/09:00 4.176
12/08/05/09:00 4.122
12/08/05/09:00 4.320
12/08/05/09:00 4.236
12/08/05/09:00 4.255
12/08/05/10:00 4.188
12/08/05/10:00 4.082
12/08/05/10:00 4.285
12/08/05/10:00 4.261
12/08/05/10:00 4.186
12/08/05/11:00 4.285
12/08/05/11:00 4.234
12/08/05/11:00 4.240
12/08/05/11:00 4.238
12/08/05/11:00 4.230
12/08/05/12:00 4.143
12/08/05/12:00 4.048
12/08/05/12:00 4.216
12/08/05/12:00 4.157
12/08/05/12:00 4.158
12/08/05/14:00 4.168
12/08/05/14:00 4.231
12/08/05/14:00 4.286
12/08/05/14:00 4.164
12/08/05/14:00 4.046
12/08/05/15:00 4.138
12/08/05/15:00 4.332
12/08/05/15:00 4.101
12/08/05/15:00 4.136
12/08/05/15:00 4.193
12/08/05/16:00 4.222
12/08/05/16:00 4.328
12/08/05/16:00 4.260
12/08/05/16:00 4.091
12/08/05/16:00 4.188
12/08/05/17:00 4.141
12/08/05/17:00 4.022
12/08/05/17:00 4.255
12/08/05/17:00 4.103
12/08/05/17:00 4.038
12/08/05/18:00 4.108
12/08/05/18:00 4.308
12/08/05/18:00 4.126
12/08/05/18:00 4.128
12/08/05/18:00 4.159
12/08/05/19:00 4.186
12/08/05/19:00 4.080
12/08/05/19:00 4.351
12/08/05/19:00 4.172
12/08/05/19:00 4.343
12/08/05/21:00 4.122
12/08/05/21:00 4.023
12/08/05/21:00 4.152
12/08/05/21:00 4.174
12/08/05/21:00 4.143
12/08/05/22:00 4.203
12/08/05/22:00 4.226
12/08/05/22:00 4.101
12/08/05/22:00 4.205
12/08/05/22:00 4.125
12/08/05/23:00 4.160
12/08/05/23:00 4.060
12/08/05/23:00 4.162
12/08/05/23:00 4.184
12/08/05/23:00 4.146
12/08/05/00:00 4.090
12/08/05/00:00 4.211
12/08/05/00:00 4.096
12/08/05/00:00 4.184
12/08/05/00:00 4.260
12/08/05/01:00 4.272
12/08/05/01:00 4.299
12/08/05/01:00 4.133
12/08/05/01:00 4.163
12/08/05/01:00 4.232
12/08/05/03:00 4.216
12/08/05/03:00 4.116
12/08/05/03:00 4.230
12/08/05/03:00 4.187
12/08/05/03:00 4.096
12/08/05/04:00 4.279
12/08/05/04:00 4.218
12/08/05/04:00 4.196
12/08/05/04:00 4.366
12/08/05/05:00 4.280
12/08/05/05:00 4.294
12/08/05/05:00 4.067
12/08/05/05:00 4.150
12/08/05/05:00 4.130
12/08/05/06:00 4.197
12/08/05/06:00 4.216
12/08/05/06:00 4.101
12/08/05/06:00 4.167
12/08/05/06:00 4.181
12/08/05/07:00 4.149
12/08/05/07:00 4.253
12/08/05/07:00 4.227
12/08/05/07:00 4.247
12/08/05/07:00 4.306
12/09/05/09:00 4.310
12/09/05/09:00 4.265
12/09/05/09:00 4.254
12/09/05/09:00 4.051
12/09/05/09:00 4.245
12/09/05/10:00 4.323
12/09/05/10:00 4.276
12/09/05/10:00 4.180
12/09/05/10:00 4.223
12/09/05/10:00 4.303
12/09/05/11:00 4.247
12/09/05/11:00 4.169
12/09/05/11:00 4.226
12/09/05/11:00 4.019
12/09/05/11:00 4.086
12/09/05/12:00 4.213
12/09/05/12:00 4.180
12/09/05/12:00 4.223
12/09/05/12:00 4.247
12/09/05/12:00 4.085
12/09/05/14:00 4.121
12/09/05/14:00 4.183
12/09/05/14:00 4.197
12/09/05/14:00 4.130
12/09/05/14:00 4.161
12/09/05/15:00 4.244
12/09/05/15:00 4.256
12/09/05/15:00 4.280
12/09/05/15:00 4.298
12/09/05/15:00 4.261