title: na.omit() or exclude=NA doesn't function
body: <p>I repeat I am new to R and did not figure out just yet all the idiosyncrasy of R. I do not know where I am missing the simple point here.</p>
  
  <p>I try to plot a histogram with ggplot and do not manage to label my x axis properly and to get rid of the NA values I have (see small data set provided): <a href="http://ubuntuone.com/7iL1UG0YxHpKs0kXN8uQhC" rel="nofollow noreferrer">http://ubuntuone.com/7iL1UG0YxHpKs0kXN8uQhC</a> (8,2 KB)</p>
  
  <pre><code>sex &lt;- factor(rawdata$Sesso)
  ggplot(rawdata, aes(x=sex)) + geom_histogram() + xlab("") + ylab("Number of Persons") + ggtitle("Gender of Persons") + scale_x_discrete(breaks=c("1", "2", NA),labels=c("Male", "Female", "NA"))
  </code></pre>
  
  <p>I get either this: <img src="https://i.stack.imgur.com/7XNF2.png" alt="wrong plot">
  or that: <img src="https://i.stack.imgur.com/bWkY9.png" alt="wrong plot2"></p>
  
  <p>This is what I tried so far to address that issue:</p>
  
  <pre><code>sex &lt;- sex[complete.cases(sex),]
  ggplot(na.omit(rawdata), aes(x=sex)) + geom_histogram() + xlab("") + ylab("Number of Persons") + ggtitle("Gender of Persons")
  ggplot(rawdata[!is.na(rawdata$Sesso),], aes(x=rawdata$Sesso)) + geom_histogram() + xlab("") + ylab("Number of Persons") + ggtitle("Gender of Persons")
  sex &lt;- factor(rawdata$Sesso,exclude=NULL)
  </code></pre>
  
  <p>But to no avail!</p>