title: Add line graph based on new data to series of boxplots
body: <p>I have used the following R script to create two side-side boxplots. One for 1999 and one for 2008:</p>
  
  <pre><code>library(tidyverse)
  mpg %&gt;% ggplot(aes(as_factor(year), hwy))+geom_boxplot()
  </code></pre>
  
  <p>I have a new data set for manufacturer XYZ that has two observations,one for 1999 and one for 2008:</p>
  
  <pre><code>manufacturer &lt;- c("xyz", "xyz")
  year &lt;- c(1999, 2008)
  hwy &lt;- c(19, 30)
  df &lt;- data.frame(manufacturer, year, hwy)
  </code></pre>
  
  <p>Is there a simple way to adding the two observations from the new data set (df) in my boxplot graph?
  I have seen a few other similar posts (e.g., <a href="https://stackoverflow.com/questions/53153701/ggplot-adding-new-data-to-the-existing-grouped-boxplot">ggplot: adding new data to the existing grouped boxplot</a>) but the problems/solutions seem to be more complicated and I could not follow them.
  Thanks</p>