title: How to add image to ggplot2 under the grid?
body: <p>The most popular (and simplest) way to adding image to the ggplot2 graph is <code>annotation_custom</code>:</p>
  
  <pre><code>library(ggplot2)
  library(png)
  library(grid)
  
  img &lt;- readPNG(system.file("img", "Rlogo.png", package="png"), TRUE)
  gpp &lt;- rasterGrob(img, interpolate=TRUE)
  gpp$width &lt;- unit(1, "npc") 
      gpp$height &lt;- unit(1, "npc")
  df &lt;- data.frame(x=seq(1,2,0.01),y=seq(1,2,0.01))
  ggplot(df,aes(x=x,y=y)) + 
      annotation_custom(gpp, xmin=1, xmax=2.5, ymin=1, ymax=1.5) +
      geom_point()
  </code></pre>
  
  <p>In this way, image will be placed over the scale grid.</p>
  
  <p>How to place image <em>under</em> the grid, but with bindings to the coords, not to the borders of the plot?</p>