Draw border around certain rows using cowplot and ggplot2

Question

I want to somehow indicate that certain rows in a multipanel figure should be compared together. For example, I want to make this plot:

Look like this plot

Answer 1

Since the result of plot_grid() is a ggplot object, one way to do this is to use nested plot grids: one plot_grid() for each row, with the appropriate border added via theme().

plot_grid(
  # row 1
  plot_grid(plot.mpg, plot.diamonds, nrow = 1, labels = c('A', 'B')) +
    theme(plot.background = element_rect(color = "black")),

  # row 2
  plot_grid(plot.mpg2, plot.diamonds2, nrow = 1, labels = c('C', 'D')) +
    theme(plot.background = element_rect(color = "black")), 

  nrow = 2)