title: How to specify decimal places not significant digits in xtable output
body: <p>I'm using the R <code>xtable</code> package to print out my dataframes as LaTeX tables. I have been using the <code>digits</code> option to specify the number of significant digits for the values in my tables. However, with <code>digits=3</code>, I get outputs like:</p>
  
  <pre><code>...
  chat &amp; chatd &amp; 1.04 &amp; -0.526 &amp; 0.943 \\ 
  dav1 &amp; davis &amp; 0.922 &amp; 0.0551 &amp; 0.374 \\ 
  ...
  </code></pre>
  
  <p>I'd like to be able to specify the number of decimal places in my tables, so that everything has 3 decimal places, even if the last few places are padded with zeros. So, for example, the first numeric column would have</p>
  
  <pre><code>1.040
  0.922
  </code></pre>
  
  <p>so that everything has 3 decimal places, and therefore everything lines up nicely.</p>
  
  <p>Is there a way to do this?</p>