geom_boxplot_group_number_and_ks_group.R

# Function to add number ontop of geom_boxplot
library(ggplot2)
library(dplyr)

# /!\ Need to manually change the "groups" if ks_test included
add_groups_and_number = function(given_plot, ks_tests) {
    y_range = ggplot_build(given_plot)$layout$panel_params[[1]]$y.range
    
    y_upper = max(y_range)
    
    # Compute table with groups from Kruskal-Wallis test
    group_df = ks_tests$groups %>%
        tibble::rownames_to_column("cyl") %>%
        select(cyl, groups) %>%
        mutate(y = y_upper)
    group_df$cyl = factor(trimws(group_df$cyl))
    
    # Function to return number of observation
    give_n = function(x, y_up = y_upper) {
        data.frame(y = y_up*1.06,
                   label = paste("n =", length(x))
        )
    }
    
    new_plot = given_plot +
        # Kruskal-Wallis group
        geom_text(data = group_df, aes(x = cyl, y = y, label = groups,
                                       fill = NULL),
                  color = "black", size = rel(5)) +
        # Give back only length of data
        stat_summary(fun.data = give_n, aes(x = as.factor(cyl)), geom = "text")
    return(new_plot)
}


plot_cyl_mpg = ggplot(mtcars, aes(x = as.factor(cyl), mpg)) +
    geom_boxplot()

ks_test = agricolae::kruskal(mtcars$mpg, trt = as.factor(mtcars$cyl))