title: Add points to pairs plot?
body: <p>Is there any way for me to add some points to a <code>pairs</code> plot?</p>
  
  <p>For example, I can plot the Iris dataset with <code>pairs(iris[1:4])</code>, but I wanted to execute a clustering method (for example, <code>kmeans</code>) over this dataset and plot its resulting centroids on the plot I already had.</p>
  
  <p>It would help too if there's a way to plot the whole data and the centroids together in a single <code>pairs</code> plot in such a way that the centroids can be plotted in a different way. The idea is, I plot <code>pairs(rbind(iris[1:4],centers)</code> (where centers are the three centroids' data) but plotting the three last elements of this matrix in a different way, like changing cex or pch. Is it possible?</p>