title: image.plot does not create new image
body: <p>Here is a sample of code that produces a image plot. But for some reason, when I re-run it, no new image is created but it overlays the existing plot. However I expect it to create a new image, certainly as it has a specific parameter 'add' to specify that it has to be added to the existing plot (but default value is FALSE). </p>
  
  <p>Does anybody know what is going on here?</p>
  
  <pre><code>require(fields)
  
  mat &lt;- matrix(runif(5*5), ncol=5) 
  
  mat[1,3] &lt;- NA
  
  image.plot(seq(1,5,1),seq(1,5,1), mat, col = tim.colors(64), legend.only=TRUE)
  par(oma=c(0,0,0,6))
  par(new = TRUE)
  image(seq(1,5,1),seq(1,5,1),matrix(1,5,5), col = gray(0.8), xlab = "", ylab = "", axes = F)
  par(new = TRUE)
  image(seq(1,5,1),seq(1,5,1),mat, xlab="Hour of the Day", ylab = "Day of the Week", col = tim.colors(64), axes = F)
  par(oma=c(0,0,0,0))
  </code></pre>
  
  <p><strong>EDIT</strong>
  I don't know much about devices etc. But I do know that (and I am working in RStudio) when I do eg first <code>plot(1:10)</code> and then <code>plot(10:1)</code> that the figure window is cleared and the second plot is shown without being added to the first one (in RStudio you have off course the additional functionality that you can also browse your previous figures, but if I do the same in a plain R console, the plotting window is cleared when the second plot is called). And <em>that</em> is what I want, I just expect that the figure window is cleared when I call image.plot a second time, instead of overlaying the image from the previous time I called image.plot.</p>