title: How to filter ggplot bar graph to only show counts above a threshold
body: <p>Say I use the following to produce the bar graph below.  How would I only visualize bars where the count is above, say, 20?  I'm able to do this kind of filtering with the which function on variables I create or that exist in my data, but I'm not sure how to access/filter the auto-counts generated by ggplot.  Thanks.</p>
  
  <pre><code>g &lt;- ggplot(mpg, aes(class))
  g + geom_bar()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/MWu3g.png" rel="nofollow noreferrer">Simple bar graph</a></p>