title: Error: Continuous value supplied to discrete scale
body: <p>I'm trying to make a heat plot for a discrete outcomes (each value uses a single color) such that:</p>
  
  <pre><code>df&lt;-data.frame(x=rep(LETTERS[1:10], each=10), 
                 y=rep(1:10,10),
                 value=sample(1:8, 100,replace=T))
  colors&lt;-c("green", "lightyellow", "yellow", "orange", "orangered", "red", "darkred", "black")
  ggplot(df, aes(x=x, y=y))+
      geom_tile(aes(fill=value), colour="white")+
      #scale_fill_gradient(low="green", high="red")
      scale_fill_manual(values=colors)
  Error: Continuous value supplied to discrete scale
  </code></pre>
  
  <p>Does somebody know how to fix it and apply the colors variable to the heatmap?</p>