Graph info

Should I use this graph?


This graph requires:

✅ two numeric (continuous) variables

Description

Standard histograms separate a variable’s values into discrete groups, or ‘bins,’ which are arranged in increasing order across the x axis. The y axis displays the frequency (or count) of values within each bin.

Vertical bars capture the variable’s distribution using the height of the bar to represent the number of values per ‘bin’, and the number of bars corresponds with the bin value (or ‘bin-width’).

When we extend this display to two numerical/quantitative variables, the bins are used to divide the total graph area into a grid, and color is used to display the variation in frequency (or count) of both variable values that fall within each intersecting square.

Getting set up

PACKAGES:

Install packages.

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

DATA:

Artwork by @allison_horst

We’ll take the flipper_length_mm, bill_length_mm, and species variables from palmerpenguins::penguins and drop the missing values.

Code 
penguins_2dhist <- palmerpenguins::penguins |> 
    dplyr::select(flipper_length_mm, bill_length_mm, 
                  sex, species, body_mass_g) |> 
    tidyr::drop_na()
glimpse(penguins_2dhist)
Rows: 333
Columns: 5
$ flipper_length_mm <int> 181, 186, 195, 193, 190, 181, 195, 182, 191, 198, 18…
$ bill_length_mm    <dbl> 39.1, 39.5, 40.3, 36.7, 39.3, 38.9, 39.2, 41.1, 38.6…
$ sex               <fct> male, female, female, female, male, female, male, fe…
$ species           <fct> Adelie, Adelie, Adelie, Adelie, Adelie, Adelie, Adel…
$ body_mass_g       <int> 3750, 3800, 3250, 3450, 3650, 3625, 4675, 3200, 3800…

The grammar

CODE:

Create labels with labs()

Initialize the graph with ggplot() and provide data

Map bill_length_mm to the x and flipper_length_mm to the y

Add the geom_bin2d() layer

Code 
labs_2dhist <- labs(
    title = "Adult Foraging Penguins", 
    subtitle = "Near Palmer Station, Antarctica", 
    x = "Bill length (mm)", 
    y = "Flipper length (mm)")

ggp2_2dhist <- ggplot(data = penguins_2dhist, 
    mapping = aes(x = bill_length_mm, 
                  y = flipper_length_mm)) + 
    geom_bin2d()
            
ggp2_2dhist + 
    labs_2dhist

GRAPH:

More info

BINS:

The value for bins will be vary depending on the variable values–there is no correct number. If the number of bins is too low, the density may hide important nuances between the variables. If the number of bins is too high, the noise might drown out the signal.

Below we change the bins to 15 and save this layer as ggp2_2dbins15:

Code 
ggp2_base <- ggplot(data = penguins_2dhist, 
    mapping = aes(x = bill_length_mm, 
                  y = flipper_length_mm)) 
ggp2_2dbins15 <- ggp2_base + 
                    geom_bin2d(bins = 15) 
ggp2_2dbins15 + 
     labs_2dhist

SCALES:

scale_fill_continuous_sequential() comes with a variety of palettes to choose from (run hcl_palettes(type = "sequential") to view the full list).

We can also reverse the order of the fill color scale with rev (TRUE or FALSE).

Code 
ggp2_2dbins15 + 
    scale_fill_continuous_sequential(
        palette = "Mako", 
        rev = TRUE) +
    labs_2dhist 
ggp2_2dbins15 + 
    scale_fill_continuous_sequential(
        palette = "Mako", 
        rev = FALSE) +
    labs_2dhist

OPTIONS:

If you set the point shape to 21, you have control over both color and fill.

In the previous example we showed how to reverse the color scale for the palette in scale_fill_continuous_sequential(). Below we reverse the color scale, but also manually set which colors on the scale we want to begin with (i.e., smallest data value) and which color we want to end with (i.e., the largest data value). Possible values range from 0 - 1.

We also add a geom_point() layer.

Code 
ggp2_2dbins15 + 
    scale_fill_continuous_sequential(
        palette = "SunsetDark",
        rev = TRUE,
        begin = 0.7, end = 0.2) +
    geom_point(color = "#007bff",
        fill = "#FFFFFF", shape = 21,
        size = 2.2, alpha = 0.75) +
    labs_2dhist