title: Add empty plots to facet, and combine with another facet
body: <p>Using this <a href="https://stackoverflow.com/questions/30372368/adding-empty-graphs-to-facet-wrap-in-ggplot2">SO solution</a> I created a facet with two "empty" plots, with the aim of combining with another group of facet_wrap plots, as shown below. The purpose is to have two y-axis labels for different unit measurements. How can I make the grid layout look like the top image, which produces the arrangement I want, but not the axis labels? This was accomplished with <code>plot_grid</code> with individual plots. My current output does not scale correctly and overlaps the other plots, as seen in the second image, but provides the axis labels.
  I have example data below, just copy and run the code to input it.
  <a href="https://i.stack.imgur.com/ZyRYH.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZyRYH.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/MhVAq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MhVAq.png" alt="enter image description here"></a></p>
  
  <pre><code>library(ggplot2)
  library(grid)
  library(cowplot)
  
  clipboard &lt;- readClipboard()
  test.data &lt;- read.table(file = "clipboard", sep = ",", header=TRUE)
  test.data1 &lt;- test.data[1:24, ]
  test.data2 &lt;- test.data[25:32, ]
  
  testplot1 &lt;- ggplot(test.data1, aes(Station, value)) +
    geom_point() +
    labs(x = "Stations", y = "Scale A") +
    theme(legend.position = "none", legend.title = element_blank()) +
    facet_wrap( ~ constituent, ncol = 3, scales = "free_y")
  
  testplot2 &lt;- ggplot(test.data2, aes(Station, value)) +
    geom_point() +
    labs(x = "Stations", y = "Scale B") +
    theme(legend.position = "none", legend.title = element_blank(), axis.title.y = element_text(hjust = 0.2)) +
    facet_wrap( ~ constituent, ncol = 1, scales = "free_y")
  
  blankplots &lt;- ggplotGrob(testplot2)
  rm_grobs &lt;- blankplots$layout$name %in% c("panel-1-1", "panel-2-1", "strip-t-1-1", "strip-t-1-2")
  blankplots$grobs[rm_grobs] &lt;- NULL
  blankplots$layout &lt;- blankplots$layout[!rm_grobs, ]
  grid.newpage()
  emptygrids &lt;- grid.draw(blankplots)
  
  plot_grid(emptygrids, MPLOOplot1)
  </code></pre>
  
  <p>Example date is below:</p>
  
  <pre><code>Station,constituent,value
  A1,A,1
  B1,A,1
  A1,B,2
  B1,B,2
  A1,C,3
  B1,C,3
  A1,D,4
  B1,D,4
  A1,E,5
  B1,E,5
  A1,F,6
  B1,F,6
  A1,G,7
  B1,G,7
  A1,H,8
  B1,H,8
  A1,I,9
  B1,I,9
  A1,J,10
  B1,J,10
  A1,K,11
  B1,K,11
  A1,L,1.4
  B1,L,1.4
  A1,Blank1,NA
  B1,Blank1,NA
  A1,Blank2,NA
  B1,Blank2,NA
  A1,XX,0.52
  B1,XX,0.52
  A1,YY,0.355
  B1,YY,0.355
  </code></pre>