4 Collective geoms

Geoms can be roughly divided into individual and collective geoms. An individual geom draws a distinct graphical object for each observation (row). For example, the point geom draws one point per row. A collective geom displays multiple observations with one geometric object. This may be a result of a statistical summary, like a boxplot, or may be fundamental to the display of the geom, like a polygon. Lines and paths fall somewhere in between: each line is composed of a set of straight segments, but each segment represents two points. How do we control the assignment of observations to graphical elements? This is the job of the group aesthetic.

By default, the group aesthetic is mapped to the interaction of all discrete variables in the plot. This often partitions the data correctly, but when it does not, or when no discrete variable is used in a plot, you’ll need to explicitly define the grouping structure by mapping group to a variable that has a different value for each group.

There are three common cases where the default is not enough, and we will consider each one below. In the following examples, we will use a simple longitudinal dataset, Oxboys, from the nlme package. It records the heights (height) and centered ages (age) of 26 boys (Subject), measured on nine occasions (Occasion). Subject and Occassion are stored as ordered factors.

data(Oxboys, package = "nlme")
#>   Subject     age height Occasion
#> 1       1 -1.0000    140        1
#> 2       1 -0.7479    143        2
#> 3       1 -0.4630    145        3
#> 4       1 -0.1643    147        4
#> 5       1 -0.0027    148        5
#> 6       1  0.2466    150        6