title: ggplot2: two time series with different and irregular date intervals in a single plot
body: <p>Using the ggplot2 package, I would like to obtain a plot that contains two time series that have data points on different dates.</p>
  
  <p>For instance, one data frame looks like:</p>
  
  <pre><code>date1, value1  
  2010-01-05, 2921.74  
  2010-01-08, 2703.89  
  2010-01-14, 3594.21  
  2010-01-20, 3659.22  
  </code></pre>
  
  <p>The other data frame looks like</p>
  
  <pre><code>date2, value2  
  2010-01-01, 285.85  
  2010-01-02, 229.20  
  2010-01-05, 333.91  
  2010-01-06, 338.27  
  2010-01-07, 272.85  
  2010-01-08, 249.04  
  2010-01-09, 240.07  
  2010-01-10, 255.06  
  2010-01-11, 275.42  
  2010-01-12, 252.39  
  </code></pre>
  
  <p>I would like to plot these two time series in one and the same plot, with date on the X axis and value on the Y axis. It's fairly easy with {base} plot, but I'd like to do it with ggplot.</p>