title: Add vertical lines to quantmod::chart_Series
body: <p>I want to add vertical lines on several dates on a certain graph. So far I haven't managed to achieve this simple task. This is what I tried: </p>
  
  <pre><code>&gt; s &lt;- get(getSymbols('nvmi'))["2012::"]
  &gt; d1 &lt;- index(s[100])
  &gt; d1
  [1] "2012-05-24"
  
  &gt; chart_Series(s,TA="addLines(v=d1)")
  Error in get.current.chob() : improperly set or missing graphics device
  
  &gt; chart_Series(s)
  &gt; abline(v=d1) 
  # nothing
  
  &gt; add_TA("addLines(v=d1")
  Error in `[.data.frame`(lenv$xdata, Env$xsubset) : 
    undefined columns selected
  </code></pre>
  
  <p>From what I have already read here, I know that <code>abline</code> is not supposed to work with the new <code>chart_Series</code> function. It doesn't seem to work anyway. The <code>addLines</code> function does not work in any of the forms I tried - plain <code>addLines</code>, <code>plot(addLines(...))</code>, <code>chart_Series(..., TA="addLines(...)")</code>  or <code>add_TA("addLines(...)")</code>.</p>
  
  <p>I need to use the experimental version of quantmod because it solved other problems I had with the old version. <code>d1</code> would eventually be a list of dates.</p>