25  Grouped box plots

This graph should be readable but is currently undergoing final polishing.

This graph requires:

✅ a categorical variable

✅ a numeric (continuous) variable

25.1 Description

Grouped box plots display five different statistical measures across a series of categories, or groups, of a discrete, ordinal, or interval variable.

The five statistical measures are 1) the first quartile, 2) the second quartile, and 3) the third quartile. The fourth and fifth values are the largest/smallest values no further than 1.5 X inter-quartile range from the hinges.

25.2 Set up

PACKAGES:

Install packages.

install.packages("palmerpenguins")
library(palmerpenguins) 
library(ggplot2)

DATA:

Artwork by Allison Horst

Remove the missing island values from the penguins data.

peng_box <- filter(penguins, !is.na(island))
glimpse(peng_box)
#> Rows: 344
#> Columns: 8
#> $ species           <fct> Adelie, Adelie, Adelie…
#> $ island            <fct> Torgersen, Torgersen, …
#> $ bill_length_mm    <dbl> 39.1, 39.5, 40.3, NA, …
#> $ bill_depth_mm     <dbl> 18.7, 17.4, 18.0, NA, …
#> $ flipper_length_mm <int> 181, 186, 195, NA, 193…
#> $ body_mass_g       <int> 3750, 3800, 3250, NA, …
#> $ sex               <fct> male, female, female, …
#> $ year              <int> 2007, 2007, 2007, 2007…

25.3 Grammar

CODE:

• Create labels with labs()

• Initialize the graph with ggplot() and provide data

• Map island to the x axis and to fill

• Map bill_length_mm to the y axis

• Add geom_boxplot() and set the alpha to 2/3

• Remove the legend with show.legend = FALSE

labs_grp_boxplots <- labs(
  title = "Adult foraging penguins", 
  subtitle = "Palmer Archipelago, Antarctica",
  x = "Island", fill = "Island",
  y = "Bill length (millimeters)")
ggp2_grp_boxplots <- ggplot(data = peng_box,
       aes(x = island, 
           y = bill_length_mm, 
        fill = island)) +
  geom_boxplot(alpha = 2/3, 
      show.legend = FALSE) 
ggp2_grp_boxplots + 
  labs_grp_boxplots

GRAPH:

When a categorical variable is supplied, the plot will contain a box for each level or group.

25.4 More info

25.4.1 Notches

Add notches to the box plot using the notch = TRUE and notchwidth arguments.

ggp2_grp_box_notch <- ggplot(data = peng_box,
       aes(x = island, 
           y = bill_length_mm, 
        fill = island)) +
  geom_boxplot(
      notch = TRUE, 
      notchwidth = 0.85, 
      alpha = 2/3, 
      show.legend = FALSE) 
ggp2_grp_box_notch + 
    labs_grp_boxplots

25.4.2 Outliers

Box plots display outliers using points, and we can change the color these using the outlier.colour argument. Inside the geom_boxplot(), we map island to color and set outlier.colour to NULL:

ggp2_grp_box_outliers <- ggplot(data = peng_box,
       aes(x = island, 
           y = bill_length_mm,
           fill = island)) +
  geom_boxplot(aes(color = island),
      outlier.colour = NULL, 
      outlier.size = 2,
      notch = TRUE, 
      notchwidth = 0.85, 
      alpha = 2/3, 
      show.legend = FALSE) 
ggp2_grp_box_outliers + 
    labs_grp_boxplots