title: Horizontal barplot in ggplot2 with faces having different categories
body: <p>I have a data frame with ids, categories, and values, while I manage easily to have an horizontal dot-plot, with products grouped by category in distinct facets,
  when I try with a barplot, event categories with missing data show up.</p>
  
  <p>Any hint? Is it a bug or am I missing some detail?</p>
  
  <p>Thanks,
  Marco.</p>
  
  <pre><code>## I have a data frame with ids, categories, and values
  d=data.frame(prd=c("orange","apple","pear","bread","crackers"),
           cat=c("fruit","fruit","fruit","bakery","bakery"),
           qty=c(10,20,15,8,17)
           )
  
  # I manage to have an horizontal dot-plot, with products grouped by category in distinct facets
  ggplot(d,aes(y=prd,x=qty)) + 
    geom_point(stat="identity",size=4) + 
    geom_segment(aes(yend=prd), xend=0, colour="grey50") +
    facet_grid(cat ~ .,scale="free",space="free") +
    theme_light()
  
  # though when I try with a barplot, bars, with missing data show up
  ggplot(d,aes(x=prd,y=qty)) + 
    geom_bar(stat="identity") + 
    coord_flip() + 
    facet_grid(cat ~ .,scale="free",space="free") +
    theme_light()
  </code></pre>