title: plotting multiple xts objects in one window
body: <p>I have found some answers to this online but for some reason am interpreting incorrectly because I cannot get it to work. My goal is to simply use the <code>xts</code> plotting feature (with the the way it creates the axis, gridlines,etc.) to plot multiple plots:</p>
  
  <pre><code>x &lt;- xts(data.frame(a=1:100, b=100:1),seq(from=as.Date("2010-01-01"), by="days", len=100))
  &gt; plot(x, screens=1)
  Warning messages:
  1: In plot.xts(x, screens = 1) :
    only the univariate series will be plotted
  2: In plot.window(...) : "screens" is not a graphical parameter
  3: In plot.xy(xy, type, ...) : "screens" is not a graphical parameter
  4: In axis(1, at = xycoords$x, labels = FALSE, col = "#BBBBBB", ...) :
    "screens" is not a graphical parameter
  5: In axis(1, at = xycoords$x[ep], labels = names(ep), las = 1, lwd = 1,  :
    "screens" is not a graphical parameter
  6: In axis(2, ...) : "screens" is not a graphical parameter
  7: In title(screens = 1) : "screens" is not a graphical parameter
  </code></pre>
  
  <p>Another try:</p>
  
  <pre><code>&gt; plot(x, plot.type="single")
  Warning messages:
  1: In plot.xts(x, plot.type = "single") :
     only the univariate series will be plotted
  2: In plot.window(...) : "plot.type" is not a graphical parameter
  3: In plot.xy(xy, type, ...) : "plot.type" is not a graphical parameter
  4: In axis(1, at = xycoords$x, labels = FALSE, col = "#BBBBBB", ...) :
    "plot.type" is not a graphical parameter
  5: In axis(1, at = xycoords$x[ep], labels = names(ep), las = 1, lwd = 1,  :
    "plot.type" is not a graphical parameter
  6: In axis(2, ...) : "plot.type" is not a graphical parameter
  7: In title(plot.type = "single") :
    "plot.type" is not a graphical parameter
  </code></pre>
  
  <p>To be clear: I can do this using <code>lines</code> but I wonder if there is a way to do this all at once.</p>