Align ggplot2 plots vertically

Here's an example to align more basic grobs,

library(ggplot2)library(grid)library(gtable)p <- qplot(1,1)g <- ggplotGrob(p)panel_id <- g$layout[g$layout$name == "panel",c("t","l")]g <- gtable_add_cols(g, unit(1,"cm"))g <- gtable_add_grob(g, rectGrob(gp=gpar(fill="red")),                     t = panel_id$t, l = ncol(g))g <- gtable_add_rows(g, unit(1,"in"), 0)g <- gtable_add_grob(g, rectGrob(gp=gpar(fill="blue")),                     t = 1, l = panel_id$l)grid.newpage()grid.draw(g)

and with your grobs

The answer from @baptiste helped me get a better understanding of the gtable structure and how to modify it. Below I post just my modified variant as code snippet for (my own) reuse.

It is using find_panel() to get the panel extent, and pipes %>% the modifications directly into grid.draw. The piping considerably simplified playing around with the gtable_* functions, as it allows to easily uncomment single lines and check the effect on the final plot.

library(ggplot2)library(grid)library(gtable)library(dplyr)p <- ggplot(tribble(~x,~y,~a,~b,                     1, 1, "a1","b1",                    1, 1, "a2","b1",                    1, 1, "a2","b2"),            aes(x=x,y=y)) +   geom_point() +   facet_grid(vars(a),vars(b))g <- ggplotGrob(p)panels_extent <- g %>% find_panel()g %>%  # Add red box to the very right, by appending a column and then filling it  gtable_add_cols(widths = unit(1,"cm"), pos = -1) %>%  gtable_add_grob(rectGrob(gp=gpar(fill="red")),                  t = panels_extent$t, b = panels_extent$b,                  l = -1, r = -1) %>%  # Add green box to the top, by prepending a row and then filling it  # Note the green box extends horizontally over the first panel as well   # as the space in between.  gtable_add_rows(heights = unit(1,"cm"), pos = 0) %>%  gtable_add_grob(rectGrob(gp=gpar(fill="green")),                  t = 1, b = 1,                  l = panels_extent$l, r = panels_extent$l+1) %>%  {grid.newpage();grid.draw(.)}