ggplot adding image on top-right in two plots with different scales

Question

I would like to add an image to two ggplots on the top-right. My problem is that the scales are different which is why I have to specify xmin, xmax... for each plot, seperately:

library(ggplot2)
library(png)
library(grid)

# subsetting
am0 <- subset(mtcars, am == 0)
am1 <- subset(mtcars, am == 1)

# ggplot
plt <- function(dat){
    p <- ggplot(dat, aes(mpg,disp)) + 
      geom_point()

    return(p)
}

p0 <- plt(am0)
p1 <- plt(am1)

# reading image
img <- readPNG(system.file("img", "Rlogo.png", package = "png"))

# adding image
p0 + annotation_custom(rasterGrob(img), xmin = 20, xmax = 25, ymin = 400, ymax = 450)
p1 + annotation_custom(rasterGrob(img), xmin = 30, xmax = 35, ymin = 300, ymax = 350)

leads to the following results:

My question is: Is there a way to add images so that the size and the position of the image is constant and not dependent on the scale of the plot? it sould look something like this:

Answer 1

We can automate the process of specifying the location and scales, so that you don't need to change the locations manually, as shown in the following example:

get.xy <- function(p) {
  g_data <- ggplot_build(p)
  data.frame(xmax = max(g_data$data[[1]]$x),
             ymax = max(g_data$data[[1]]$y),
             xmin = min(g_data$data[[1]]$x),
             ymin = min(g_data$data[[1]]$y))
}

# this returns the dataframe with required x, y params for annotation_custom,
# ensuring the size and position of the image constant
get.params.df <- function(p0, p1, width, height) {
  df0 <- cbind.data.frame(get.xy(p0), width=width, height=height)
  df1 <- cbind.data.frame(get.xy(p1))
  df1$width <- df0$width*(df1$xmax-df1$xmin)/(df0$xmax-df0$xmin)
  df1$height <- df0$height*(df1$ymax-df1$ymin)/(df0$ymax-df0$ymin)
  df <- rbind(df0, df1)
  return(data.frame(xmin=df$xmax-df$width, xmax=df$xmax+df$width, ymin=df$ymax-df$height, ymax=df$ymax+df$height))
}

p0 <- plt(am0)
p1 <- plt(am1)

df <- get.params.df(p0, p1, width=10, height=10)

# adding image
library(gridExtra)
grid.arrange(
  p0 + annotation_custom(rasterGrob(img), xmin=df[1,1],xmax=df[1,2], ymin=df[1,3], ymax=df[1,4]),
  p1 + annotation_custom(rasterGrob(img), xmin=df[2,1],xmax=df[2,2], ymin=df[2,3], ymax=df[2,4])  
)

If you want bigger image change the width height parameter only, everything else remains unchanged.

df <- get.params.df(p0, p1, width=25, height=25)
library(gridExtra)
grid.arrange(
  p0 + annotation_custom(rasterGrob(img), xmin=df[1,1],xmax=df[1,2], ymin=df[1,3], ymax=df[1,4]),
  p1 + annotation_custom(rasterGrob(img), xmin=df[2,1],xmax=df[2,2], ymin=df[2,3], ymax=df[2,4])  
)