title: horizontal dashed or dotted lines in xtable
body: <p>I wonder if there is a way to make some hlines in xtable dashed or dotted using command <code>hline.after</code>. I couldn't find a solution.</p>
  
  <p>MWE (Rnw):</p>
  
  <pre><code>\documentclass{article}
  \usepackage{booktabs}
  \begin{document}
  
  &lt;&lt;mytable, results='asis', echo=F&gt;&gt;=
  library("xtable")
  print(xtable(matrix(rnorm(32), 8,4), align="lllll"), booktabs=T, hline.after = c(-1,0,4,8))
  @
  
  \end{document}
  </code></pre>
  
  <p>gives this latex table code:</p>
  
  <pre><code>\begin{table}[ht]
  \centering
  \begin{tabular}{lllll}
  \toprule 
   &amp; 1 &amp; 2 &amp; 3 &amp; 4 \\ 
   \midrule
   1 &amp; 0.50 &amp; -0.49 &amp; -1.32 &amp; -0.29 \\ 
   2 &amp; 1.01 &amp; 0.57 &amp; 1.35 &amp; 0.54 \\ 
   3 &amp; -1.20 &amp; 1.02 &amp; 1.12 &amp; 0.07 \\ 
   4 &amp; 0.04 &amp; 0.14 &amp; 0.23 &amp; -0.38 \\ 
    \midrule
   5 &amp; -1.71 &amp; -0.90 &amp; 0.59 &amp; -0.05 \\ 
   6 &amp; 0.14 &amp; -0.82 &amp; -0.44 &amp; 0.52 \\ 
   7 &amp; -0.28 &amp; 1.91 &amp; -1.80 &amp; 0.53 \\ 
   8 &amp; 0.38 &amp; 2.66 &amp; 0.26 &amp; -1.38 \\ 
   \bottomrule
   \end{tabular}
   \end{table}
  </code></pre>
  
  <p>Instead of the solid <code>\midrule</code> after line 4, I'd like a dashed or, preferably, dotted line.</p>
  
  <p>I know, in latex dashed horizontal lines can me made using <a href="http://www.ctan.org/pkg/arydshln" rel="nofollow">arydshln</a> package, also dotted hlines are possible manipulating \dashlinedash{}, \dashlinegap{}
  and \arrayrulewidth{}.</p>
  
  <p>But how can I make dotted/dashed lines when printing an xtable? Does anyone have a solution?</p>