`ggplot2 - facet_grid`: Axes without ticks in interior facets - r

Question

I would like to create facet_grid / facet_wrap plot with the x axis being repeated under each graph but with ticks present only on the lowest graph.
Here is an example of a plot with the x axis present only once using facet_grid
ggplot(mtcars, aes(y=mpg,x=cyl)) +
facet_grid(am~., scales="free") +
geom_point() +
theme_classic() +
theme(strip.background = element_rect(colour="white", fill="white"),
strip.text.y = element_blank())
Here is an example of a plot with the x axis present twice but with ticks both times using facet_wrap
ggplot(mtcars, aes(y=mpg, x=cyl)) +
facet_wrap(~am, ncol=1, scales="free") +
geom_point() +
theme_classic() +
theme(strip.background = element_rect(colour="white", fill="white"),
strip.text.x = element_blank())
I would like the same plot as the one just above but without the ticks on the x-axis of the upper graph. Or if you prefer, I would like the same plot as the first one but with an x-axis on the upper graph.

Answer 1

This is a very verbose solution, but I don't think you can get the plot you want using just the usual ggplot functions.
library(ggplot2)
library(grid)
Plot <- ggplot(mtcars, aes(y=mpg, x=cyl)) +
facet_wrap(~am, ncol=1, scales="free") +
geom_point() +
theme_classic() +
theme(strip.background = element_rect(colour="white", fill="white"),
strip.text.x = element_blank())
Switching off the top x-axis requires modifying the gtable object for the plot.
Plot.build <- ggplot_gtable(ggplot_build(Plot))
axis.pos <- grep("axis-b-1-1", Plot.build$layout$name)
num.ticks <- length(Plot.build$grobs[[axis.pos]]$children[2]$axis$grobs[[1]]$y)
This step removes the axis labels:
Plot.build$grobs[[axis.pos]]$children$axis$grobs[[2]]$children[[1]]$label <- rep("", num.ticks)
This step removes the tick marks:
Plot.build$grobs[[axes.pos]]$children[2]$axis$grobs[[1]]$y <- rep(unit(0, units="cm"), num.ticks)
Finally, the plot is generated using:
grid.draw(Plot.build)

Answer 2

The workaround I use to get just an axis line (no tick marks) is to use geom_hline() to fake an axis.
#make a dataframe with the y minimum for each facet
fake.axes <- data.frame(mpg = c(10, 15), #y minimum to use for axis location
am = c(0,1)) #facetting variable
#add an "axis" without ticks to upper graph using geom_hline()
ggplot(mtcars, aes(y=mpg,x=cyl)) +
facet_grid(am~., scales="free") +
geom_point() +
geom_hline(aes(yintercept = mpg), fake.axes, #dataframe with fake axes
linetype = c("solid", "blank")) + #line for top graph, blank for bottom graph
theme_classic() +
theme(strip.background = element_rect(colour="white", fill="white"),
strip.text.y = element_blank())
If you haven't used scales = "free", and all the axes are in the same location this is even simpler, you can skip making a dataframe with yintercepts for each facet and simply add
geom_hline(yintercept = 10) (or whatever your minimum is) to your plot code to add an axis line on each facet.