Force x-axis labels on facet_grid ggplot: x-axis labels differ per row

Question

I was so happy to find the greater part of a solution to my question in the post, "Force X axis text on for all facets of a facet_grid plot".

I'd like to create a graph to look somewhat like the OP Drew Steen's, except I have more than two rows of facets, and I'd like to make the x-axes labels different for each row.

I made a super-hacky solution out of @baptiste's awesome answer (mostly because I am unfamiliar with the gtable package), and I'd like to know:

1. If there is a more elegant solution than the mess I wrote below
2. How to insert labels for the "Premium" (middle) row.

Here is the code I adapted from @Drew Steen and @baptiste,

library(ggplot2)

diamondSub <-subset(diamonds, (cut=="Ideal" | cut=="Premium" | cut == "Very Good") & (color=="E" | color=="I"))

p<- ggplot(diamondSub, aes(x=carat, y=price)) + 
  geom_blank()+ 
  geom_point() +
  scale_x_discrete(breaks=c(1, 2, 3, 4), labels=c("a", "b", "c", "d")) +
  facet_grid(cut~color, scales="free_x")
p

p2<- ggplot(diamondSub, aes(x=carat, y=price)) + 
  geom_blank()+ 
  geom_point() +
  scale_x_discrete(breaks=c(1, 2, 3, 4), labels=c("f", "g", "h", "i")) +
  facet_grid(cut~color, scales="free_x")
p2

library(gtable)

g <- ggplotGrob(p)
g2 <- ggplotGrob(p2)

# locate the panels
panels <- grep("panel", g$layout$name)
panels2 <- grep("panel", g2$layout$name)

top <- unique(g$layout$t[panels])
top2 <- unique(g2$layout$t[panels2])
# intersperse a copy of the bottom axes
all <- gtable:::rbind_gtable(gtable:::rbind_gtable(g[seq.int(min(top)), ], 
                                                   g[max(top)+1,], "first"), 
                             g2[seq(min(top2)+1, nrow(g2)),], "first")
grid.newpage()
grid.draw(all)

Answer 1

Alternative Version (added 24 April 2015) showing manual construction of the elements and containing more commenting.

## Alternative version
library(gtable)
library(grid)

# Get the ggplot grobs
g <- ggplotGrob(p)
g2 <- ggplotGrob(p2)

# Show the layout.
# Note the rows and columns
# In this case, the g2 layout is the same as the g layout
gtable_show_layout(g)

# The large panels are the plot panels.
# We will need rows 4, 6 and 8.

# For the top "Very Good" row, we will also need rows 1, 2, and 3.

# The axis is located in row 9

# Therefore rbind the grob in rows 1, 2, 3, and 4, with the grob in row 9.
top.row <- rbind(g[1:4, ], g[9, ], size = "first")

# The second "Premium" row
# We need the panels in row 6 plus the small gap row above,
# and rbind that to the axis
middle.row = rbind(g2[5:6, ], g2[9,], size = "first")

# The bottom "Ideal" row
# We need the panel in row 8 plus the small gap in the row above
# plus the axis in the row below
# plus rows below that (axis label and margin)
bottom.row = g2[7:11, ]


# rbind the three rows 
all <- rbind(rbind(top.row, middle.row, size = "first"), bottom.row, size = "first")

# Draw it
grid.newpage()
grid.draw(all)

# Maybe add a little more space between the rows
gtable_show_layout(all)
all$heights[c(6,9)] = unit(1, "lines")

# Draw it
grid.newpage()
grid.draw(all)

But you don't even need to do the binding of the panel to the axis; just select the appropriate rows from the layout:

top.row <- g[c(1:4, 9), ]
middle.row = g2[c(5:6, 9), ]
bottom.row = g2[7:11, ]

then bind these three new rows.

Original Version

It is only in the binding of the rows that you have made a mistake. You have bound an axis to the top "Very Good" row. The bottom "Ideal" row already has an axis, so no binding is required. To Fix: The middle "Premium" row needs an axis.

I have constructed each row separately, binding an axis to the top and middle rows, then binding the three rows.

I've added another step to add a little more space between the rows.

g <- ggplotGrob(p)
g2 <- ggplotGrob(p2)

# locate the panels
panels <- grep("panel", g$layout$name)
panels2 <- grep("panel", g2$layout$name)

top <- unique(g$layout$t[panels])
top2 <- unique(g2$layout$t[panels2])

# Construct each row separately
top.row <- gtable:::rbind_gtable(g[seq.int(min(top)), ], g[max(top)+1,], "first")
middle.row <- gtable:::rbind_gtable(g2[c(top[2]-1,top[2]), ], g2[max(top)+1,], "first")
bottom.row <- g2[(max(top2)-1):nrow(g2), ]

all <- gtable:::rbind_gtable(gtable:::rbind_gtable(top.row, middle.row, "first"), bottom.row, "first")

# Draw it
grid.newpage()
grid.draw(all)

# Maybe add a little more space between the rows
all$heights[c(6,9)] = unit(1, "lines")

grid.newpage()
grid.draw(all)