Summarize y values for each unique x. Either a fun.data measure (mean ± CI,
median hilow, …) or three independent scalar SummaryFuns (y / ymin / ymax).
A fun.data-style summary: from a group’s values it returns (y, ymin, ymax)
together — needed for measures where the interval depends on the centre (mean
± CI), which the scalar SummaryFuns cannot express. These mirror R’s
Hmisc helpers used by ggplot2::stat_summary.