title: hide a legend with ggplot
body: <p>Suppose for example I have</p>
  
  <pre><code>ggplot(data=PlantGrowth, aes(x=group, y=weight, fill=group)) + geom_boxplot()
  </code></pre>
  
  <p>and I don't want to display the legend. What is the simplest way to hide or remove the legend from a plot?</p>