title: Setting colour of a line in a ggplot2 expression with multiple aes() arguments
body: <p>I would like to plot a different line for each of my series in this data set:</p>
  
  <pre><code>example &lt;- data.frame(xAxis = c(1, 2, 3, 4, 5),
                    ValueA = c(5.5, 4.5, 4, 2.9, 2),
                    ValueB = c(5, 5.3, 5, 4.7, 4),
                    ValueC = c(4, 3.2, 3, 4, 3),
                    ValueD = c(5, 4.5, 3, 2.9, 3))
  </code></pre>
  
  <p>Following what seems like the intended usage of <code>geom_line</code> and <code>aes</code> in the ggplot2 package, I construct my graph like so:</p>
  
  <pre><code>library(ggplot2)
  
  ggplot(example, aes(x = xAxis)) + 
      geom_line(aes(y = ValueA)) + 
      geom_line(aes(y = ValueB)) + 
      geom_line(aes(y = ValueC)) + 
      geom_line(aes(y = ValueD))
  </code></pre>
  
  <p>Setting the colour argument though is creating problems. Using the following seems to label the series, but not affect the colour selection:</p>
  
  <pre><code>ggplot(example, aes(x = xAxis)) + 
  geom_line(aes(y = ValueA, colour = "green")) + 
  geom_line(aes(y = ValueB, colour = "blue")) + 
  geom_line(aes(y = ValueC, colour = "yellow")) + 
  geom_line(aes(y = ValueD, colour = "red"))
  </code></pre>
  
  <p>However, if I set each of them to "red", then the plot understands to set them all to "red". </p>
  
  <pre><code>ggplot(example, aes(x = xAxis)) + 
  geom_line(aes(y = ValueA, colour = "red")) + 
  geom_line(aes(y = ValueB, colour = "red")) + 
  geom_line(aes(y = ValueC, colour = "red")) + 
  geom_line(aes(y = ValueD, colour = "red"))
  </code></pre>
  
  <p>What have I missed? I've seen that a 'standard' answer to multiple series plots in ggplot is to also use reshape to melt the data, but I feel there should be a decent solution here without needing that.</p>