Overlapping bar graphs

Graph info

Should I use this graph?


This graph requires:

✅ a numeric (continuous) variable (optional)

✅ a categorical variable

Desription

Overlapping bar graphs display counts for categorical levels, resulting in bars differentiated by color and ‘stacked’ on top of each other.

In ggplot2, we can build overlapping bar graphs using the fill argument in geom_bar() or geom_col()

Getting set up

PACKAGES:

Install packages.

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

DATA:

Artwork by @allison_horst

Remove missing species from penguins and filter the data to only penguins on "Dream" island.

Code 
penguins_ovrlp <- filter(penguins,
                      !is.na(species) & 
                            island == "Dream")
glimpse(penguins_ovrlp)
Rows: 124
Columns: 8
$ species           <fct> Adelie, Adelie, Adelie, Adelie, Adelie, Adelie, Adel…
$ island            <fct> Dream, Dream, Dream, Dream, Dream, Dream, Dream, Dre…
$ bill_length_mm    <dbl> 39.5, 37.2, 39.5, 40.9, 36.4, 39.2, 38.8, 42.2, 37.6…
$ bill_depth_mm     <dbl> 16.7, 18.1, 17.8, 18.9, 17.0, 21.1, 20.0, 18.5, 19.3…
$ flipper_length_mm <int> 178, 178, 188, 184, 195, 196, 190, 180, 181, 184, 18…
$ body_mass_g       <int> 3250, 3900, 3300, 3900, 3325, 4150, 3950, 3550, 3300…
$ sex               <fct> female, male, female, male, female, male, male, fema…
$ year              <int> 2007, 2007, 2007, 2007, 2007, 2007, 2007, 2007, 2007…

The grammar

CODE:

Create labels with labs()

Initialize the graph with ggplot() and provide data

Map flipper_length_mm to the x and species to fill

Add the geom_bar() layer

Code 
labs_bar_ovrlp <- labs(
  title = "Adult foraging penguins on Dream island",
  x = "Flipper length (mm)",
  y = "Count",
  fill = "Species")
ggp2_bar_ovrlp <- ggplot(data = penguins_ovrlp,
          aes(x = flipper_length_mm, fill = species)) +
                geom_bar() 
ggp2_bar_ovrlp + 
  labs_bar_ovrlp

GRAPH:

More info

Overlapping bar graphs can also be built with geom_col().

geom_bar() has additional options for arranging overlapping bars. We can set the position argument to "dodge" or "dodge2, depending on how we’d like the data displayed.

geom_col():

To build an overlapping bar graph with geom_col(), we need to create a column with the counts for flipper_length_mm and species in the dataset.

Create the penguins_col data:

Code 
penguins_col <- penguins_ovrlp |>
    count(species, flipper_length_mm, name = "Count")

Map the counts to the y axis, flipper_length_mm to the x axis, and species to fill.

Code 
labs_col_ovrlp <- labs(
  title = "Adult foraging penguins on Dream island",
  subtitle = "built with 'geom_col()'",
  x = "Flipper length (mm)",
  y = "Count",
  fill = "Species")
ggp2_col_ovrlp <- ggplot(data = penguins_col, 
    mapping = aes(y = Count, 
        x = flipper_length_mm, 
        fill = species)) + 
    geom_col()
ggp2_col_ovrlp + 
    labs_col_ovrlp

Compare the two graphs below:

  • position = "dodge" preserves the vertical position of a geom while adjusting the horizontal position

  • requires the grouping variable to be be specified in the global or ⁠geom_⁠ layer

dodge:

Create the penguins_dodge data.

Code 
penguins_dodge <- filter(penguins,
                      !is.na(species) & 
                        !is.na(sex) &
                            island == "Dream")
glimpse(penguins_dodge)
Rows: 123
Columns: 8
$ species           <fct> Adelie, Adelie, Adelie, Adelie, Adelie, Adelie, Adel…
$ island            <fct> Dream, Dream, Dream, Dream, Dream, Dream, Dream, Dre…
$ bill_length_mm    <dbl> 39.5, 37.2, 39.5, 40.9, 36.4, 39.2, 38.8, 42.2, 37.6…
$ bill_depth_mm     <dbl> 16.7, 18.1, 17.8, 18.9, 17.0, 21.1, 20.0, 18.5, 19.3…
$ flipper_length_mm <int> 178, 178, 188, 184, 195, 196, 190, 180, 181, 184, 18…
$ body_mass_g       <int> 3250, 3900, 3300, 3900, 3325, 4150, 3950, 3550, 3300…
$ sex               <fct> female, male, female, male, female, male, male, fema…
$ year              <int> 2007, 2007, 2007, 2007, 2007, 2007, 2007, 2007, 2007…

Create labels with labs()

Initialize the graph with ggplot() and provide data

Map species to the x and island to group and fill

Inside the geom_bar() function, set position to "dodge"

Code 
labs_bar_dodge <- labs(
  title = "Adult foraging penguins on Dream island",
  subtitle = "position = 'dodge'",
  x = "Sex",
  y = "Count",
  fill = "Species")
ggp2_bar_dodge <- ggplot(data = penguins_dodge,
                    aes(x = sex, 
                        group = species, 
                        fill = species)) +
                    geom_bar(
                        position = "dodge")
ggp2_bar_dodge +
  labs_bar_dodge

dodge2:

  • works without a grouping variable in a layer

  • works with bars and rectangles

  • useful for arranging graphs with variable widths.

Create labels with labs()

Initialize the graph with ggplot() and provide data

Map species to x and island to fill

Inside geom_bar(), set position to "dodge2"

Code 
labs_bar_dodge2 <- labs(
  title = "Adult foraging penguins on Dream island",
  subtitle = "position = 'dodge2'",
  x = "Sex",
  y = "Count",
  fill = "Species")
ggp2_bar_dodge2 <- ggplot(data = penguins_dodge,
                    aes(x = sex, 
                        fill = species)) +
                    geom_bar(
                        position = "dodge2")
ggp2_bar_dodge2 +
  labs_bar_dodge2

Compare the two graphs below: