title: R: Save multiple plots from a file list into a single file (png or pdf or other format)
body: <p>I have more than 10 files (in the end some hundreds...). which I generated in R in png format saved into a folder.</p>
  
  <p><strong>My question</strong>: How could I save these files into a multiplot (e.g. 4 figures on one page arranged in 2 rows and 2 columns)? </p>
  
  <p>I know that this is possible to incorporate inside a plot loop by using <code>par(mfrow=c(2,2))</code> but how could I do this outside just calling the files in the folder after they are generated? </p>