Show outlier labels ggplot and geom_boxplot r for multiple variables?

Question

Any suggestions on how to solve problem? Unlike other similar questions here on the channel that presented the solution for a variable as a factor, my case is different. I would like to see the labels of the outliers for multiple variables.

I have the following chart as below. for example:

It was created with this command:

z_mtcars <-data.frame(scale(mtcars[-12]))
z_mtcars$type<-rownames(mtcars)
z_mtcars %>% melt(id.vars = "type")  %>%
ggplot() +
aes( x = variable, y = value, fill = as.numeric(variable)) +
geom_boxplot() +
scale_fill_distiller(palette = "Blues") +
scale_alpha(range = c(1,1)) +
ggtitle("Boxplot: Standardized Score (Z-Scale) ") +
xlab("Variables") +
ylab("Value") + 
labs(fill = "Order of 
Variables") +
theme_classic() +
theme(axis.text.x = element_text(angle = 90,hjust = 1)) +
geom_hline(yintercept = 0, linetype = "dashed", color = "red") +
geom_hline(yintercept = 1, linetype = "dotted", color = "blue") +
theme(legend.position = "left")

user_3304471 · Answer

Here is what I tried. I simplified your code a bit to highlight the point you are asking. You want to somehow find label information of the outliers. You can identify outliers using the borrowed function below. When you identify them, you add car names in a new column called outlier. You use this information in geom_text_repel() in the ggrepel package.

library(tidyverse)
library(ggrepel)

z_mtcars <- data.frame(scale(mtcars[-12]))
z_mtcars$type <- rownames(mtcars)

I borrowed this function from this question. Credit goes to JasonAizkalns.

is_outlier <- function(x) {
  return(x < quantile(x, 0.25) - 1.5 * IQR(x) | x > quantile(x, 0.75) + 1.5 * IQR(x))
}

z_mtcars %>%
pivot_longer(names_to = "variable", values_to = "value", -type) %>% 
group_by(variable) %>% 
mutate(outlier = if_else(is_outlier(value), type, NA_character_)) %>% 
ggplot(aes(x = variable, y = value, color = variable)) +
geom_boxplot() +
geom_text_repel(aes(label = outlier), na.rm = TRUE, show.legend = F)

user_496488 · Answer

In the code below, we use geom_text to add labels to the outliers. Within geom_text, we calculate the outlier locations and filter the data down to the outliers. One odd thing is that I had to set coef to 1.4 in boxplot.stats (instead of the default 1.5) in order to get all of the outliers included. Not sure why. I've also switched to pivot_longer from dplyr, since reshape2 is outdated, and kept only the relevant portions of code.

library(tidyverse)
  
z_mtcars <- data.frame(scale(mtcars[-12]))
z_mtcars$type <- rownames(mtcars)

z_mtcars %>% 
  pivot_longer(cols=-type) %>% 
  ggplot(aes(x = name, y = value)) +
    geom_boxplot() +
    geom_text(data=. %>% 
                group_by(name) %>%  
                filter(value %in% boxplot.stats(value, coef=1.4)$out),
              aes(label=type, y=value), nudge_x=0.1, colour="red", size=3, hjust=0) +
    theme_classic() +
    expand_limits(x=12.6)

Show outlier labels ggplot and geom_boxplot r for multiple variables?

2 Answers