11  Heatmaps

This graph requires:

✅ two numeric (continuous) variables

✅ a categorical variable

11.1 Description

Heatmaps are color graphs that show data as a matrix with categories on the x and y axes. Each cell’s color corresponds to its value. They are useful for showing magnitude in two dimensions and often include a color scale. The intersecting cells contain variations of color saturation (i.e., the grade of purity or vividness) to represent the numerical values between groups.

Heatmap legends should be positioned on top or bottom and justified horizontally to preserve shape and improve readability.

11.2 Set up

PACKAGES:

Install packages.

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

DATA:

For the heatmap, we’re going to re-structure and filter the bob_ross data from the fivethirtyeight package.

heatmap_ross <- fivethirtyeight::bob_ross |>
  pivot_longer(-c(episode, season,
    episode_num, title),
    names_to = "object",
    values_to = "present") |>
  mutate(present = as.logical(present),
    object = str_replace_all(object, "_", " ")) |>
  arrange(episode, object) |> 
  filter(object %in% c("conifer", "trees", 
    "tree", "snow", "palm trees", "grass", 
    "flowers", "cactus", "bushes", "cirrus", 
    "cumulus", "deciduous", "clouds", "fog")) |>
  group_by(season, object) |>
  summarise(occurrences = sum(present)) |>
  ungroup()
#> `summarise()` has grouped output by 'season'. You
#> can override using the `.groups` argument.

11.3 Grammar

CODE:

• Create labels with labs()

• Initialize the graph with ggplot() and provide data

• Assign season to x, object to y, and occurrences to fill

• Add the geom_tile()

• Move the legend to the bottom with theme(legend.position = "bottom")

labs_heatmap_tile <- labs(
  title = "Bob Ross' plants & clouds", 
  x = "Episode", 
  y = "Plant & Cloud Objects", 
  fill = "Occurrences")
ggp2_heatmap_tile <- ggplot(data = heatmap_ross, 
       aes(x = season, 
           y = object, 
           fill = occurrences)) + 
       geom_tile() + 
    theme(legend.position = "bottom")
ggp2_heatmap_tile + 
  labs_heatmap_tile

GRAPH:

11.4 More info

In addition to geom_tile(), heatmaps can also be created with the geom_raster() function.

11.4.1 geom_raster()

• Create labels with labs()

• Initialize the graph with ggplot() and provide data

• Assign season to x, object to y, and occurrences to fill

• Add the geom_raster()

• Move the legend to the bottom with theme(legend.position = "bottom")

labs_heatmap_raster <- labs(
  title = "Bob Ross' plants & clouds", 
  x = "Episode", 
  y = "Plant/Cloud Object", 
  fill = "Occurrences")
ggp2_heatmap_raster <- ggplot(data = heatmap_ross, 
       aes(x = season, 
           y = object, 
           fill = occurrences)) + 
       geom_raster() + 
    theme(legend.position = "bottom")
ggp2_heatmap_raster + 
  labs_heatmap_raster