title: Heatmap type plot with ggplot 2
body: <p>For data of the following type, in a dataframe called df.test</p>
  
  <pre><code>ID    value
  A1    -2
  A2     0
  A3     3
  A4     2
  </code></pre>
  
  <p>How do I make a heatmap-type plot in ggplot2 so that I get one row with each of the values (they're discrete, but ggplot and fill require me to use them continuously) marked by a different colour or a shade?</p>
  
  <pre><code>ggplot(df.test, aes(x=ID, y=value)) + geom_tile(aes(fill=value)) 
  </code></pre>
  
  <p>gets the colour right, but there are multiple rows corresponding to each value. </p>