title: scale_fill_discrete and scale_fill_manual - legend options confusion
body: <p>I have a problem with applying two different changes on my <code>ggplot2</code> object. I have a barplot with 3 groups on it. I want to: </p>
  
  <ol>
  <li>set color for each group</li>
  <li>reorder the order of legend labels</li>
  </ol>
  
  <p>I have found the following solutions to the above problems: </p>
  
  <ol>
  <li><code>+ scale_fill_manual(values = c("red", "green", "blue"))</code></li>
  <li><code>+ scale_fill_discrete(guide = guide_legend(reverse=TRUE))</code></li>
  </ol>
  
  <p>The problem is the second application (<code>+ scale_fill_discrete(...)</code>) is overriding the first one:</p>
  
  <pre><code>Scale for 'fill' is already present. Adding another scale for 'fill', which will replace the existing scale.
  </code></pre>
  
  <p>(separately both of them work fine). How to deal with it?  </p>