Prepares data for significance staples on ggplot

Usage

sig_seg(df, compare.var, y.var, group.var, heights, ends)

df: Aggregated data frame
compare.var: Indicator of categories compared
y.var: Means or other compared quantities (used to define the ends of the staples)
group.var: Any additional variable in the dataset (e.g., groupuing variable)
heights: Numeric vector. Height or width of each staple.
ends: Numeric vector. Fixed ends for even staple graphs, in case specified, `y.var` is ignored.