Drawing the keys of a ggplot2 legend from SVG files

ggplot_svg_keys.R

# Load libraries
library(ggplot2)
library(grid)
library(rlang)
library(grImport2)

# Replace by intended SVG file
file <- "https://upload.wikimedia.org/wikipedia/commons/d/db/Brain_Drawing.svg"
download.file(file, tmp <- tempfile(fileext = ".svg"))

# Function factory for rendering keys from an SVG file
key_svg <- function(file) {
  
  # Import SVG file as a grob
  grob <- readPicture(rawToChar(rsvg::rsvg_svg(file)))
  grob <- pictureGrob(grob)
  
  # Key drawing function
  function(data, params, size) {
    
    # Determine graphical parameters
    gp <- gpar(
      col  = alpha(data$fill %||% "black", data$alpha),
      fill = alpha(data$fill %||% "black", data$alpha),
      lwd  = (data$stroke %||% data$size %||% 0.5) * .pt,
      fontsize = 1
    )
    
    # Edit graphical parameters of imported SVG
    editGrob(grob, gp = gp, global = TRUE,
             gPath = "picComplexPath", grep = TRUE)
  }
}

# Plot
ggplot(mpg, aes(displ, hwy, fill = class)) +
  geom_point(shape = 21, key_glyph = key_svg(tmp)) +
  theme(legend.key.size = unit(1, "cm"))