title: Overlay grid rather than draw on top of it
body: <p>I have a ggplot2 graph which appears as follows:</p>
  
  <p><a href="https://i.stack.imgur.com/xyk5G.png" rel="noreferrer"><img src="https://i.stack.imgur.com/xyk5G.png" alt="ggplot2 graph without axis lines visible"></a></p>
  
  <p>Notice that the grid or axis lines do not show through the ribbons. One way of dealing with this is to alter the <code>alpha</code> property of the ribbons; however, this can make the lighter colours too light.</p>
  
  <p>An alternative idea would be to draw the grid/axis lines on top of the ribbons rather than beneath them. How can I achieve this render ordering?</p>
  
  <p>Naturally, the question could be asked of <em>any</em> plot generated by ggplot2. But a copy-and-paste command that illustrates the issue is as follows:</p>
  
  <pre><code> ggplot(data.frame(x=sample(1:100),y=sample(1:100)),aes(x=x,y=y))+geom_point(size=20)
  </code></pre>