title: Plotting the correlation between specific columns in a matrix using "corrgram"
body: <p>Assuming that I have matrix M = (A|B|C) where A,B, and C are columns in M. </p>
  
  <p>Using the <code>corrgram</code> function of the <code>corrgram</code> package in R, how can I plot the correlation between A, in the one hand, and B and C on the other hand? In other words I'm not interested in plotting all the pairwise correlations between the three columns, but only: <code>corr(A,B)</code> and <code>corr(A,C)</code></p>