Specify variables that define separate groups of rows on which binning is to be performed, or by which to create different plots.
BY variables are optional (in most processes) variable specifications whose values define groups of
observations, such as hour, month, or year. BY variables can enable analyses to be performed separately on different groups as defined by that variable. Choosing a BY variable such as
Mass, for example, groups the observations by mass and conducts separate analyses at each value of mass.