GGally reexports three ggplot2
statistics functions from ggstats. Please see their
corresponding vignettes in ggstats for more details: * Compute
cross-tabulation statistics with stat_cross()
(vignette("stat_cross", "ggstats")) * Compute
custom proportions with stat_prop()
(vignette("stat_prop", "ggstats")) * Compute
weighted mean with stat_weighted_mean()
(vignette("stat_weighted_mean", "ggstats"))