Control chart limit is not a straight line

[#1]

I have many parameters, some parameters had the UCL label, but not all the parameters, example of the below chart, the UCL label is not showing, also red line had rectangle shape in and out. Why is this? Are there features to turn on and off to fix this? 

Thanks lot

[#2 · Accepted solution]

I'll wager a guess you do not have a constant subgroup size.  You probably "assigned" the subgroup by some variable and it was different for that one subgroup.  This is also why there is no number given as it depends on the subgroup.

As a side note, you really don't want to have varying subgroup sizes as this defeats the purpose of control chart method (to identify which sources of variation have greatest effect).

[#3]

Thanks, that is indeed why.