Optional factor or character vector defining groups for
individuals. When supplied, group-specific colours are used and, if
ellipse = TRUE, confidence ellipses are drawn for each group.
ellipse
Logical; draw group confidence ellipses when groups are
provided.
ellipse_level
Confidence level for group ellipses (between 0 and 1).
ellipse_n
Number of points used to draw each ellipse.
group_col
Optional vector of colours for the groups. Recycled as
needed.