Explicitly set panel size (not just plot size) in ggplot2

Question

Is it possible to explicitly set the panel size (i.e., the gray grid panel) in a ggplot? I imagine (but can't find) that there is some ggplot extension that allows for arguments that resemble panel.width = unit(3, "in"), panel.height = unit(4, "in").

I have seen solutions for setting the size of the entire plot, or of getting multiple plots to align using the egg package. But nothing that would let me explicitly set the size of the panel.

library(dplyr)
library(ggplot2)
library(tibble)

ds_mt <- mtcars %>% rownames_to_column("model")
mt_short <- ds_mt %>% arrange(nchar(model)) %>% slice(1:4)
mt_long <- ds_mt %>% arrange(-nchar(model)) %>% slice(1:4)

p_short <- 
    mt_short %>% 
    ggplot(aes(x = model, y = mpg)) + 
    geom_col() + 
    coord_flip()

p_short

Answer 1

You can use set_panel_size() function from the egg package

library(tibble)
library(dplyr)
library(ggplot2)

ds_mt <- mtcars %>% rownames_to_column("model")
mt_short <- ds_mt %>% arrange(nchar(model)) %>% slice(1:4)
mt_long <- ds_mt %>% arrange(-nchar(model)) %>% slice(1:4)

p_short <- 
  mt_short %>% 
  ggplot(aes(x = model, y = mpg)) + 
  geom_col() + 
  coord_flip()

library(egg)
library(grid)
p_fixed <- set_panel_size(p_short,
                          width  = unit(10, "cm"),
                          height = unit(4, "in"))
grid.newpage()
grid.draw(p_fixed)

^{Created on 2018-11-13 by the reprex package (v0.2.1.9000)}

Answer 2

The ggh4x package has a similar function to the egg solution presented in the other answer. A slight convenience is that the plot is still a valid ggplot after using the function, so it would work with ggsave() and other layers can be added afterwards. (disclaimer: I wrote ggh4x)

library(dplyr)
library(ggplot2)
library(tibble)
library(ggh4x)

ds_mt <- mtcars %>% rownames_to_column("model")
mt_short <- ds_mt %>% arrange(nchar(model)) %>% slice(1:4)
mt_long <- ds_mt %>% arrange(-nchar(model)) %>% slice(1:4)

mt_short %>% 
  ggplot(aes(x = model, y = mpg)) + 
  geom_col() + 
  coord_flip() +
  force_panelsizes(rows = unit(4, "in"),
                   cols = unit(3, "in"))

^{Created on 2021-04-21 by the reprex package (v1.0.0)}