Programmatically position image on ggplot using annotation_custom

Question

I am trying to create a function (inside an R package) that adds an image to the lower right corner of a ggplot. Since this will be included in a R package, I want this function to programmatically position this image to the bottom right of the plot, without any user intervention.

Here is what I have so far:

# Setup
library(tidyverse)

get_png <- function(filename) {
  grid::rasterGrob(png::readPNG(filename), interpolate = TRUE)
}

# DaSL logo
l <- get_png("logo.png")

# Main function
add_dasl_logo <- function(logo) {
  list(annotation_custom(logo, xmin = 5, xmax = 7.5, ymin = 0, ymax = 15), 
       coord_cartesian(clip = "off"),
       theme(plot.margin = unit(c(1, 1, 3, 1), "lines"))
  )
}

# Plot
ggplot(mpg, aes(displ, hwy, colour = hwy)) +
  geom_point() +
  add_dasl_logo(l)

Image file (logo.png):

Right now, the x-y min/max locations are manually set for this specific ggplot. I don't expect the average ggplot2 user to figure out the xmin, xmax, ymin, ymax coordinates.

How do I programmatically find these coordinates and use them in add_dasl_logo() without any user intervention?

Answer 1

Instead of setting the position for your logo in data coordinates via annotation_custom you could set the position in relative coordinates directly in grid::rasterGrob. This way the logo will be placed at the same position and with the same height and width for each plot.

# Setup
library(tidyverse)

get_png <- function(filename) {
  grid::rasterGrob(png::readPNG(filename),
    interpolate = TRUE,
    x = unit(1, "npc") + unit(10, "pt"),
    y = unit(0, "npc") - unit(20, "pt"),
    height = unit(10, "pt"),
    hjust = 1,
    vjust = 1
  )
}

download.file("https://i.stack.imgur.com/uj2rP.png", "logo.png")

# DaSL logo
l <- get_png("logo.png")

# Main function
add_dasl_logo <- function(logo) {
  list(
    annotation_custom(logo),
    coord_cartesian(clip = "off"),
    theme(plot.margin = unit(c(1, 1, 3, 1), "lines"))
  )
}

ggplot(mpg, aes(displ, hwy, colour = hwy)) +
  geom_point() +
  add_dasl_logo(l)

ggplot(mpg, aes(cty, hwy, colour = hwy)) +
  geom_point() +
  add_dasl_logo(l)