title: How can I fill under a line that I plotted with type='s'
body: <p>I would like to fill the area under a step function line I ploted using <code>plot(X, type='s')</code> I tried <code>polygon</code> with no success.</p>
  
  <pre><code>set.seed(1);y = abs(rnorm(10))
  plot(y,ylim=c(0,max(y)), type="p",pch=24)
  lines(y, type='s')
  abline(h=0)
  </code></pre>
  
  <p>Say I want to paint in gray under the curve and above <code>y=0</code>
  <img src="https://i.stack.imgur.com/oHAoW.png" alt="enter image description here"></p>