title: List of data.frames to list of xts objects
body: <p>I have a <code>data.frame</code> with 72 monthly time series stacked in long form (vertical). I've  used split to create a <code>list</code> of <code>data.frames</code> for each series. I now want to take each <code>data.frame</code> and convert it to an <code>xts</code> object, using the <code>value</code> and <code>date</code> contained within the dataframe. The list has 72 dataframes and I would like a returned list of 72 xts objects. The list looks like:</p>
  
  <pre><code>LAUPS55030003:'data.frame': 282 obs. of  6 variables:
   ..$ series_id     : chr [1:282] "LAUPS55030003" "LAUPS55030003" "LAUPS55030003"    
   ...
   ..$ year          : int [1:282] 1990 1990 1990 1990 1990 1990 1990 1990 1990 1990 ...
   ..$ period        : chr [1:282] "01" "02" "03" "04" ...
    ..$ value         : num [1:282] 12.6 5.3 5.3 4.7 4.7 4.7 5.5 4.6 4.6 4.3 ...
    ..$ footnote_codes: chr [1:282] "" "" "" "" ...
    ..$ date          : Date[1:282], format: "1990-01-01" "1990-02-01" "1990-03-01" "1990-0 
  </code></pre>
  
  <p>I've tried using llply and lapply to no avail or even ddply. I get errors when trying to write the as.xts function. I think I'm on the right track but my syntax is apparently off.</p>
  
  <p>I've also tried:</p>
  
  <pre><code>lx &lt;- dlply(ur, .(ur$series_id), .fun=(as.xts), ur[,4], order.by=ur[,6])
  </code></pre>
  
  <p>Pointers?</p>