title: ggplot missing labels in legend
body: <p>I want to plot some data and then add a regression line and horizontal lines for the mean and mean +/- the standard deviation, each in a different colour. I have managed to do this but I can't get the legend right. It shows labels for the first two but says nothing about the three horizontal lines. How can I get the legend to have a label for each of these three lines too? </p>
  
  <p>Another post mentioned show_guide = TRUE, but this didn't work for me. I've also been looking at the documentation for scale_colour_manual but that didn't help me.</p>
  
  <p>My code is this:</p>
  
  <pre><code>p &lt;- qplot(mpg, wt, data = mtcars, colour="1")
  p &lt;- p + geom_smooth(method='lm',aes(x=mpg,y=wt,colour="2"),formula=y~x)
  p &lt;- p + geom_hline(yintercept = mean(mtcars$wt), colour = "3")
  p &lt;- p + geom_hline(yintercept = mean(mtcars$wt) + sd(mtcars$wt), colour = "3",    linetype="dashed")
  p &lt;- p + geom_hline(yintercept = mean(mtcars$wt) - sd(mtcars$wt), colour = "3", linetype="dashed")      
  p &lt;- p + labs(colour="")    
  p &lt;- p + scale_colour_manual(values = c("red","blue", "green","green","green"),labels=c("Data","Regression","Mean","Mean + SD","Mean - SD"))
  p &lt;- p + guides(colour = guide_legend())
  print(p)
  </code></pre>
  
  <p>Any help will be much appreciated,
  cheers!</p>