title: How do you draw a boxplot without specifying x axis?
body: <p>The base graphics can nicely plot a boxplot using a simple command</p>
  
  <pre><code>data(mtcars)
  boxplot(mtcars$mpg)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/59siY.png" alt="enter image description here"></p>
  
  <p>But <code>qplot</code> requires y axis. How can I achieve with qplot the same like base graphics boxplot and not get this error?</p>
  
  <pre><code>qplot(mtcars$mpg,geom='boxplot')
  Error: stat_boxplot requires the following missing aesthetics: y
  </code></pre>