title: Lattice, xyplot, 2 time series on same graph (overlay)
body: <p>I want to plot two time series on the same graph, using xyplot.</p>
  
  <p>Here is an example of what I mean using ts.plot()</p>
  
  <pre><code>a1 = ts(rnorm(20), start=c(1980,1), freq=4)
  a2 = a1+3
  ts.plot(a1,a2)
  </code></pre>
  
  <p>I have tried </p>
  
  <p><code>xyplot(merge(a1,a2))</code></p>
  
  <p>which returns an error, and I have tried</p>
  
  <pre><code>xyplot(merge(as.xts(a1),as.xts(a2)))
  </code></pre>
  
  <p>which plots them in different panels, not on the same panel.</p>