title: Display custom image as geom_point
body: <p>Is it possible to display custom image (say png format) as geom_point in R ggplot? </p>
  
  <pre><code>library(png)
  pic1 &lt;- readPNG("pic1.png")
  
  png("Heatmap.png", units="px", width=3200, height=3200, res=300)
  ggplot(data_frame, aes(medium, day, fill = Transactions))  +
     geom_tile(colour="white")  +
     facet_grid(dime3_year~dime3_month) + 
     scale_fill_gradient(high="blue",low="white") +
     theme_bw() + 
     geom_point(aes(dime3_channel, day, size=Conv,alpha=Conv,image=(annotation_raster(pic1,xmin=0,ymin=0,xmax=5,ymax=5)),color="firebrick")) +
  </code></pre>
  
  <p>Gives error:</p>
  
  <blockquote>
    <p>Don't know how to automatically pick scale for object of type
    proto/environment. Defaulting to continuous Error: Aesthetics must
    either be length one, or the same length as the
    dataProblems:(annotation_raster(conv_pic, xmin = 0, ymin = 0, xmax =
    5, ymax = 5))</p>
  </blockquote>