Insert Image/PNG ggplot2 - Cowplot

I'm trying to use cowplot package's draw_image() function. I've managed to get an image in a graph as an example.

I can't work out how the xy locations work, I had to keep inputting random numbers until I got to see the image.

require(ggplot2) #required packages
require(cowplot)
require(magick)

p1 <- qplot(Sepal.Length, Sepal.Width, data = iris)

ggdraw(p1) +      
  draw_image(
    "https://upload.wikimedia.org/wikipedia/en/7/77/EricCartman.png",
    y = 0.2,
    scale = 0.5
  )

Can anyone advise on what scale they function? It sure doesn't seem to be the same scale as the graph.

Solution

It's in the coordinates of the plot. The point that may be confusing you is that ggdraw() is setting up a new coordinate system running from 0 to 1 in both x and y. If you want to draw an image in the plot coordinates, there's no need to use ggdraw(). Just add draw_plot() directly to the plot.

library(ggplot2)
library(cowplot)
theme_set(theme_bw())

p1 <- qplot(Sepal.Length, Sepal.Width, data = iris)

# ggdraw() sets up a new coordinate system running from 0 to 1. This
# allows you to place an image on top of the plot.
ggdraw(p1) + 
  draw_image("https://upload.wikimedia.org/wikipedia/en/7/77/EricCartman.png")

# if you want to draw the image into the plot, don't use ggdraw()
p1 + draw_image(
  "https://upload.wikimedia.org/wikipedia/en/7/77/EricCartman.png",
  x = 5, y = 2.5, width = 2, height = 1.5
)

^{Created on 2018-12-19 by the reprex package (v0.2.1)}