title: Turning off some legends in a ggplot
body: <p>Suppose I have a ggplot with more than one legend.</p>
  
  <pre><code>mov &lt;- subset(movies, length != "")
  (p0 &lt;- ggplot(mov, aes(year, rating, colour = length, shape = mpaa)) +
    geom_point()
  )
  </code></pre>
  
  <p>I can turn off the display of all the legends like this:</p>
  
  <pre><code>(p1 &lt;- p0 + theme(legend.position = "none"))
  </code></pre>
  
  <p>Passing <code>show_guide = FALSE</code> to <code>geom_point</code> (as per <a href="https://stackoverflow.com/questions/12746523/preventing-a-second-legend">this question</a>) turns off the shape legend.</p>
  
  <pre><code>(p2 &lt;- ggplot(mov, aes(year, rating, colour = length, shape = mpaa)) +
    geom_point(show_guide = FALSE)
  )
  </code></pre>
  
  <p>But what if I want to turn off the colour legend instead?  There doesn't seem to be a way of telling <code>show_guide</code> which legend to apply its behaviour to.  And there is no <code>show_guide</code> argument for scales or aesthetics.</p>
  
  <pre><code>(p3 &lt;- ggplot(mov, aes(year, rating, colour = length, shape = mpaa)) +
    scale_colour_discrete(show_guide = FALSE) +
    geom_point()
  )
  # Error in discrete_scale
  
  (p4 &lt;- ggplot(mov, aes(year, rating, shape = mpaa)) +
    aes(colour = length, show_guide = FALSE) +
    geom_point()
  )
  #draws both legends
  </code></pre>
  
  <p><a href="https://stackoverflow.com/questions/11393123/controlling-ggplot2-legend-display-order">This question</a> suggests that the modern (since ggplot2 v0.9.2) way of controlling legends is with the <code>guides</code> function.</p>
  
  <p>I want to be able to do something like</p>
  
  <pre><code>p0 + guides(
    colour = guide_legend(show = FALSE) 
  )
  </code></pre>
  
  <p>but <code>guide_legend</code> doesn't have a show argument.</p>
  
  <p>How do I specify which legends get displayed?</p>