title: R: adding an histogram to a pairs plot
body: <p>I would like to add a small histogram of X4 somewhere in the upper part of the plot below. Can you help me?</p>
  
  <pre><code>X1 &lt;- runif(100)
  X2 &lt;- runif(100)
  X3 &lt;- runif(100)
  X4 &lt;- runif(100)
  pairs(~ X1 + X2 + X3, upper.panel=NULL)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/iJwmB.png" alt="enter image description here"></p>