title: R adding infrequent date 'events' to a time series plot
body: <p>I am just starting on R - and have hit a bit of a deadlock with some time series data.</p>
  
  <p>I have a time series (date and value) in 'zoo' format, that I want to annotate with a cross when an event occurs. The events are irregular and in a csv format (just the dates, sometimes repeated).</p>
  
  <p>I've managed to read in the dates etc into a format that R accepts - but i cant seem to get a means to chart the main time series with the secondary events annotated on top?</p>
  
  <p><strong>Update:</strong> 
  Sorry I missed this out earlier - below is the sort of data i am working with:</p>
  
  <pre><code>price &lt;- get.hist.quote(instrument = "msft", quote = c("Cl", "Vol"))
  </code></pre>
  
  <p>I now want to compare number of tweets (for a search term) against this, but i only have spotty data of the form:</p>
  
  <pre><code>"February 28, 2010"
  "February 20, 2010"
  "February 20, 2010"
  "August 21, 2009"
  </code></pre>
  
  <p>Some are repeated. 
  So far i've managed to write up a python script to do some cleaning (i.e. a tuple of date, occurrences), but i was hoping I could just work with the raw data using R ?</p>
  
  <p>Many thanks</p>