title: How to color cells by value in a data.frame?
body: <p>I have a data.frame called "fdr". It looks like this:</p>
  
  <pre><code>         pi     pd      aa      ef   
  gene1   0.78   0.04   0.89    0.01
  gene2   0.06   0.95   0.02    0.03
  gene3   0.98   0.07   0.03    0.23
  </code></pre>
  
  <p>Now I want to color all cells which are smaller than 0.05 in red.
  How can I do that?</p>