title: rollmean with dplyr and magrittr
body: <p>Given the following data:</p>
  
  <pre><code>    set.seed(1)
      data &lt;- data.frame(o=c('a','a','a','a','b','b','b','b','c','c','c','c'), t=c(1,2,3,4,1,2,3,4,1,2,3,4), u=runif(12), v=runif(12))
      data
         o t          u         v
      1  a 1 0.26550866 0.6870228
      2  a 2 0.37212390 0.3841037
      3  a 3 0.57285336 0.7698414
      4  a 4 0.90820779 0.4976992
      5  b 1 0.20168193 0.7176185
      6  b 2 0.89838968 0.9919061
      7  b 3 0.94467527 0.3800352
      8  b 4 0.66079779 0.7774452
      9  c 1 0.62911404 0.9347052
      10 c 2 0.06178627 0.2121425
      11 c 3 0.20597457 0.6516738
      12 c 4 0.17655675 0.1255551
  </code></pre>
  
  <p>I want to calculate the rolling mean (package zoo) of u per group defined by the coloumn o. The order for the rolling mean is set by t. The rolling mean should be added as a new column to the data.frame.</p>
  
  <p>I want to use magrittr and dplyr. I tried</p>
  
  <pre><code>    data %&gt;%
        group_by(o) %&gt;%
        sort(t) %&gt;%
        select(u) %&gt;%
        rollmean(3) %&gt;%
        rbind
  </code></pre>
  
  <p>But this won't work. Is it possible to do it with magrittr and dplyr or do I have to do it step by step?
  The values of o and t are variable in my real data. </p>
  
  <p>How do I fill the first two rows?</p>