title: ggplot2: How to use same colors in different plots for same factor
body: <p>How can I pin the same color to a value in diffent plots? </p>
  
  <p>Say I have two data.frames df1 and df2:</p>
  
  <pre><code>library(ggplot2)
  library(gridExtra)
  
  set.seed(1)
  df1 &lt;- data.frame(c=c('a', 'b', 'c', 'd', 'e'), x=1:5,  y=runif(5))
  df2 &lt;- data.frame(c=c('a', 'c', 'e', 'g', 'h'), x=1:5,  y=runif(5))
  </code></pre>
  
  <p>When plotting them using c as color-indicator I get the same five colors. </p>
  
  <pre><code>g1 &lt;- ggplot(df1, aes(x=x, y=y, fill=c)) + geom_bar(stat="identity")
  g2 &lt;- ggplot(df2, aes(x=x, y=y, fill=c)) + geom_bar(stat="identity")
  grid.arrange(g1, g2, ncol=2)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Z5l3W.png" alt="enter image description here"></p>
  
  <p>But I want that same values of c get the same color. </p>