If the subgroup-variable is numeric, the observations must be sorted in increasing order of the values of the subgroup variable. If you use a BY statement, first sort by the BY variables and then by the subgroup variable.
The unformatted values of the subgroup-variable are used to scale the horizontal axis of the control chart, and the formatted values are used to label the major tick marks on the horizontal axis. As a result, the horizontal distance between two points corresponding to consecutive subgroups is proportional to the difference between their unformatted subgroup values.
If a DATE, DATETIME, WEEKDATE, or WORDDATE format is associated with the subgroup variable, the major tick mark labels are split and displayed in two levels to save space. You can override this default with the TURNHLABELS option (which turns the labels vertically) or with tick label options in an AXISn statement specified with the HAXIS= option.