title: concatenating/ merging time series (in R)
body: <p>I have to xts/zoo objects. each has measures of different variables over a different time span. I want to create a single time series comprising all measures at all times, with NAs for missing dates/variable combinations. how do I do that?
  artificial example:</p>
  
  <pre><code>library(xts)
  x&lt;-cbind(a=1:3,b=3:1)
  rownames(x) = as.character(Sys.Date()-1:3)
  
  y&lt;-cbind(a=5:7,c=3:1)
  rownames(y) = as.character(Sys.Date()-5:7)
  
  xs=as.xts(x)
  ys=as.xts(y)
  
  #now what?
  
  #desired outcome looks like:
              a  b   c
  2013-03-10  7 NA   1
  2013-03-11  6 NA   2
  2013-03-12  5 NA   3
  2013-03-14  3  1  NA
  2013-03-15  2  2  NA
  2013-03-16  1  3  NA
  
  # regular merge looks like that (adding an a.1 variable)
  merge(xs,ys)
              a  b a.1  c
  2013-03-10 NA NA   7  1
  2013-03-11 NA NA   6  2
  2013-03-12 NA NA   5  3
  2013-03-14  3  1  NA NA
  2013-03-15  2  2  NA NA
  2013-03-16  1  3  NA NA
  
  # simple concatenation ignores variable names and looks like that
  c(xs,ys)
             a b
  2013-03-10 7 1
  2013-03-11 6 2
  2013-03-12 5 3
  2013-03-14 3 1
  2013-03-15 2 2
  2013-03-16 1 3
  
  # so what should I do?
  </code></pre>