title: ggplot2: use different colors in different facets
body: <p>I have what seems to be a very basic problem, but I cannot solve it, as I have barely used ggplots2... I just want that the plot on the left uses the colors in the variable color1 and the plot on the right uses the colors in the variable color2. This is a MWE:</p>
  
  <pre><code>library(reshape2)
  library(ggplot2)
  
  a.df &lt;- data.frame(
    id=c("a","b","c","d","e","f","g","h"), 
    var1=c(1,2,3,4,5,6,7,8), var2=c(21,22,23,24,25,26,27,28), 
    var3=c(56,57,58,59,60,61,62,63), 
    color1=c(1,2,"NONE","NONE",1,2,2,1), 
    color2=c(1,"NONE",1,1,2,2,"NONE",2)
  )
  
  a.dfm &lt;- melt(a.df, measure.vars=c("var2","var3"))
  
  ggplot(a.dfm, aes(x=value, y=var1, color=color1)) + 
    geom_point(shape=1) + 
    facet_grid(. ~ variable)
  </code></pre>
  
  <p>Thanks a lot!</p>