========== Ideas for future development ===============

 * for AffyBatch, the spatial plots could show the affyPLM residuals instead 
   of the intensities themselves. However, currently, I don't seem to be able to access
   the image method from affyPLM, and it looks messy. 
 
 * duplicate correlation: If featureData contains a factor that
   identifies features containing the same reporter sequence, we can
   check for that (scatterplots, standard deviation)