title: Plot lower triangle in a seaborn Pairgrid
body: <p>I'm a bit struggling with seaborn Pairgrid. </p>
  
  <p>Let's say I have a Pairgrid like this:</p>
  
  <p><a href="https://i.stack.imgur.com/X4c4I.png" rel="noreferrer"><img src="https://i.stack.imgur.com/X4c4I.png" alt="enter image description here"></a></p>
  
  <p>As you can see, the upper triangle plots mirror the lower triangle ones.
  I'd like to be able to plot only the lower triangle plots, but I found no easy way so far to do it. Can you help me?</p>