title: Change colours of ggplot2 (R) using column names
body: <p>I have a data frame (see below) and I want to change the colors of the plot in ggplot2 so that all the A's are colored red, B's blue, C's black, and D's yellow. </p>
  
  <pre><code>myDF
  
  label    A    B    C    D
  lab1 0.69 0.65 0.73 0.71
  lab2 0.43 0.41 0.47 0.41
  lab3 0.53 0.47 0.57 0.53
  lab4 0.55 0.47 0.60 0.55
  lab5 0.53 0.47 0.58 0.53
  </code></pre>
  
  <p>I tried the following:</p>
  
  <pre><code>p &lt;- ggplot(myDF, aes(x=label)) 
       + geom_point(aes(y=A))
       + geom_point(aes(y=B))
       + geom_point(aes(y=C))
       + geom_point(aes(y=D))
       + scale_colour_manual(values=c("A"="red", "B"= "blue", "C"="black", "D"="yellow"))
  </code></pre>
  
  <p>But it gives me an error. If I don't add the scale_colour_manual part, it gives the correct plot but all black dots. How can I do this? Thanks in advance!</p>