title: Can boxplot in base R display 'NA' when the grouping factor has missing values?
body: <p>I want this: </p>
  
  <p><img src="https://i.stack.imgur.com/xr0S0.png" alt="a boxplot with NA as a category name"></p>
  
  <p>And I thought that passing <code>na.action=na.pass</code> to boxplot would let NA show up in 
  the grouping names. Here is some sample code:  </p>
  
  <pre><code>#Build a fake dataset
  set.seed(212012)
  nn = 100
  sample_data &lt;- data.frame( score = c( rpois(nn, 1), rpois(nn, 2),
                                        rpois(nn, 1.5), rpois(nn, 3)),
                            category = c( rep(0, nn), rep(1, nn), 
                                          rep(2, nn), rep(NA, nn) ))   
  
  boxplot( score ~ category, data=sample_data, na.action=na.pass )
  </code></pre>
  
  <p>But that produces this: </p>
  
  <p><img src="https://i.stack.imgur.com/vuujA.png" alt="enter image description here"></p>
  
  <p>The 'simple' way to get what I want is the following code, but it's not great for exploratory data analysis: </p>
  
  <pre><code>sample_data$category2 &lt;- sample_data$category
  sample_data$category2[ is.na(sample_data$category) ] &lt;- 'NA'
  boxplot( score ~ category2, data=sample_data )
  </code></pre>
  
  <p>Any hints from the R Guru's out there? I was able to find out about na.pass from <a href="https://stackoverflow.com/questions/5557676/what-are-the-options-for-na-action-in-boxplot">this more general discussion</a>, and the origin of na.pass from <a href="https://stat.ethz.ch/pipermail/r-devel/2004-May/029589.html" rel="nofollow noreferrer">Prof. Ripley here</a>. But there doesn't seem to be a distinction between missing data (NA's) appearing in the data that will be split by the factor and missing data in the factor itself. Am I missing something simple, or is this more of a feature request? </p>