For use with stat_summaryh. Corresponding function for
vertical geoms is mean_se()
mean_se_h(x, mult = 1)A data frame with columns x, xmin, and xmax.
x <- rnorm(100)
mean_se_h(x)
#> x xmin xmax
#> 1 -0.01718971 -0.1131641 0.07878466