title: quality control charts handling of NA values
body: <p>I have come across quality control charts, my question is how does the qcc package handle missing values i.e.    </p>
  
  <pre><code>library(qcc)
  diameters &lt;- as.data.frame(replicate(2, rnorm(10,mean=1.31,sd=0.05)))
  diameters
  diameters[1,1]&lt;-NA
  q &lt;- qcc(diameters, type="R", nsigmas=3)
  q &lt;- qcc(diameters, type="xbar.one", nsigmas=3)
  </code></pre>
  
  <p>When plotting the R chart it makes sense to me to get an error, but why when using a xbar.one chart? Is there a way to handle NA, like not showing this points at all instead of giving an error?</p>