title: How to add a factor level on the x-axis that represents all the observations in ggplot2?
body: <p>It's easy to create plots where the x-axis has each of the factor levels as follows:</p>
  
  <pre><code>df &lt;- data.frame(value = rnorm(100), group = rep(1:3, length=100))
  ggplot() + geom_boxplot(aes(factor(group), value), data=df)
  </code></pre>
  
  <p>I want to add another factor level on the x-axis that uses data from the entire sample (instead of from only one group). A manual way to do is to <code>rbind</code> the data frame to itself as follows:</p>
  
  <pre><code>df2 &lt;- rbind(df, df)
  df2$group[100:200] &lt;- "entire sample"
  ggplot() + geom_boxplot(aes(factor(group), value), data=df2)
  </code></pre>
  
  <p>However, sometimes my data frame is quite complex and I want to avoid duplicating my data frame as such. Is there a better way?</p>