title: Removed data points in geom_bar histogram
body: <p>I’m new to R, but learning all I can. I receive the message below when plotting a facetted density plot histogram. </p>
  
  <blockquote>
    <p>Warning message: Removed ### rows containing missing values
    (geom_bar).</p>
  </blockquote>
  
  <p>I’ve read the message may be due to an x-axis issue not showing all data points, however, after investigation it doesn’t appear so. </p>
  
  <p>The error seems to be in the "position = "fill" " section of the geom_histogram. When the position = "fill"  is removed no errors are produced. </p>
  
  <p>Any help or advice is greatly appreciated.</p>
  
  <pre><code>library(ggplot2)
  
  df &lt;- data.frame(recividism = sample(0:2, 100, replace = T),
                   TotalDays = sample(15:1000, 100, replace = T),
                   NumEnroll = sample(1:7, 100, replace = T))
  
  df$recividism &lt;- as.factor(df$recividism)
  
  levels(df$recividism) &lt;- c("Not Perm", "Perm", "Rec")
  
  ggplot(data = df, aes(x = TotalDays)) + 
    geom_histogram(aes(fill = recividism), position = "fill" ) + 
    facet_grid(facets = NumEnroll ~ .)
  </code></pre>