title: How to plot a line across a four faceted ggplot bar graph with data from another dataset
body: <p>So I am trying to graph enrollment rates for subgroups within race using bar charts. I have four &quot;cohorts&quot; - an overall cohort, and then the class of 2012, 2013, and 2014. Each grid is one of the four cohorts. I have a dataset with each of the subgroup enrollment rates for each cohort . Then, I have a dataset with the overall rate of enrollment for each cohort. I want the subgroup rates to bars but I want to plot the overall rate as a single line across all bars so you can see what the overall rate was for the overall cohort, class of 2012, 2013, and 2014. I can't seem to figure out how to plot a single line for the overall rate. Each cohort's overall rate is different so I can't just plot the same line across all four grids. Here's what I have. Test has the subgroup rates. Test2 have the overall rates</p>
  <pre><code>g &lt;- ggplot(test, aes(x=Group, y= Rate, fill= Group)) +
    geom_bar(stat = &quot;identity&quot;) + 
    geom_line(test2, mapping = (aes(x= Group, y = Rate))) +
    facet_grid(factor(test$Cohort))
  </code></pre>