title: Shade part of an R plot
body: <p>I am trying to make a plot in R that has a portion of the plot grey to emphasize this area.  Unlike other examples, I don't want to color an area under a plot, but instead color an area on a plot starting at one area and going to the end of the graph.  When I try to use rect() or polygon() it obscures the plots I want to emphasize.</p>
  
  <p>For example:</p>
  
  <pre><code>x_mean &lt;- c(1, 2, 3, 4)
  y_mean &lt;- c(1, 1, 1, 1)
  
  y_max &lt;- c(4, 4, 4, 4)
  y_min &lt;- c(-4, -4, -4, -4)
  
  
  x_shade &lt;- c(2, 3, 4)
  
  y_max_shade &lt;- c(4, 4, 4)
  y_min_shade &lt;- c(-4, -4, -4)
  
  plot(x=rep(x_mean, 3), y=c(y_mean, y_max, y_min), bty='n', type="n" )
  arrows(x0=x_mean, y0=y_min, x1=x_mean, y1=y_max, length=0)
  points( x=x_mean, y=y_mean, pch=16)
  </code></pre>
  
  <p>This will plot 4 lines on the graph.  How do I draw a grey box in the background from the 2nd line to the end of the plot?</p>