title: R - add title to images with rasterImage
body: <p>I have 12 twelve PNG files which I want to combine in a single plot with 4x3 grid in R.</p>
  
  <p>So far I can create the grid with,</p>
  
  <pre><code>plot(c(0,4), c(0,3), type = "n", xaxt = "n", yaxt = "n", xlab = "", ylab = "")
  </code></pre>
  
  <p>and I can add images to it with,</p>
  
  <pre><code>rasterImage(readPNG("image1.png"), 0, 3, 1, 2)
  rasterImage(readPNG("image2.png"), 1, 3, 2, 2)
  </code></pre>
  
  <p>etc.</p>
  
  <p>I get what I want, but I also want to add a title to each image in the plot. Like image1 should have <strong>a. Image1</strong> and image2 should have <strong>b. Image2</strong> on top of the images. Is there a way to do in R?</p>
  
  <p>Thanks in advance. </p>