========== 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)