title: Add second grouping to a grouped multi-panel barchart with a shared x-axis in ggplot2
body: <p>I tried to create a second grouping with a facet grid on top of an already existing plot:</p>
  <p><a href="https://i.stack.imgur.com/VmPzY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VmPzY.png" alt="enter image description here" /></a></p>
  <p>But if I use facet_grid twice, the second one overrides the first facet_grid call.</p>
  <p><a href="https://i.stack.imgur.com/BGJtZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/BGJtZ.png" alt="enter image description here" /></a></p>
  <p>Is there a way to get the be able to use the two facet_grids for a shared x-axis?</p>