Stacking multiple figures together in ggplot - r

Question

I am attempting to make publication ready figures where the bottom axis (with tick marks) of one figure is cleanly combined with the top axis of the figure below it. Here is an example of what it might look like, although this one doesn't have tick marks on each panel:
Here is my attempt to do so, by simply using grid.arrange:
#Libraries:
library(ggplot2)
library(dplyr)
library(gridExtra)
#Filter to create two separate data sets:
dna1 <- DNase %>% filter(Run == 1)
dna2 <- DNase %>% filter(Run == 2)
#Figure 1:
dna1_plot <- ggplot(dna1, aes(x = conc, y = density)) + geom_point() + theme_classic() +
theme(axis.title.x = element_blank())
#Figure 2:
dna2_plot <- ggplot(dna2, aes(x = conc, y = density)) + geom_point() + theme_classic()
#Using grid.arrange to combine:
dna <- grid.arrange(dna1_plot, dna2_plot, nrow = 2)
And an attempt with some adjustments to the plot margins, although this didn't seem to work:
dna1_plot_round2 <- ggplot(dna1, aes(x = conc, y = density)) + geom_point() + theme_classic() +
theme(axis.title.x = element_blank(),
plot.margin = (0,0,0,0), "cm")
dna2_plot_round2 <- ggplot(dna2, aes(x = conc, y = density)) + geom_point() + theme_classic() +
theme(plot.margin = unit(c(-0.5,-1,0,0), "cm"))
dna_round2 <- grid.arrange(dna1_plot_round2, dna2_plot_round2, nrow = 2)
Does anyone know the best way to stack figures like this in ggplot? Is there a better way than using grid.arrange? If possible it would be great to see how to do it with/without tick marks on each x axis as well.
Thank you!

Answer 1

You don't need any non-native ggplot stuff. Keep your data in one data frame and use facet_grid.
dna <- DNase %>% filter(Run %in% 1:2)
ggplot(dna, aes(x = conc, y = density)) +
geom_point() +
theme_bw() +
facet_grid(rows = vars(Run)) +
theme(panel.spacing = unit(0, "mm"))

Answer 2

The R package deeptime has a function called ggarrange2 that can achieve this. Instead of just pasting the plots together like grid.arrange (and ggarrange), it lines up all of the axes and axis labels from all of the plots.
# remove bottom axis elements, reduce bottom margin, add panel border
dna1_plot_round2 <- ggplot(dna1, aes(x = conc, y = density)) + geom_point() + theme_classic() +
theme(axis.text.x = element_blank(), axis.ticks.x = element_blank(), axis.title.x = element_blank(),
plot.margin = margin(0,0,-.05,0, "cm"), panel.border = element_rect(fill = NA))
# reduce top margin (split the difference so the plots are the same height), add panel border
dna2_plot_round2 <- ggplot(dna2, aes(x = conc, y = density)) + geom_point() + theme_classic() +
theme(plot.margin = margin(-.05,0,0,0, "cm"), panel.border = element_rect(fill = NA))
dna_round2 <- ggarrange2(dna1_plot_round2, dna2_plot_round2, nrow = 2)
You might also try the fairly recent patchwork package, although I don't have much experience with it.
Note that while Gregor's answer may be fine for this specific example, this answer might be more appropriate for other folks that come across this question (and see the example at the top of the question).

Answer 3

For your purposes, I believe Gregor Thomas' answer is best. But if you are in a situation where facets aren't the best option for combining two plots, the newish package {{patchwork}} handles this more elegantly than any alternatives I've seen.
Patchwork also provides lots of options for adding annotations surrounding the combined plot. The readME and vignettes will get you started.
library(patchwork)
(dna1_plot / dna2_plot) +
plot_annotation(title = "Main title for combined plots")
Edit to better address #Cameron's question.
According to the package creator, {{patchwork}} does not add any space between the plots. The white space in the example above is due to the margins around each individual ggplot. These margins can be adjusted using the plot.margin argument in theme(), which takes a numeric vector of the top, right, bottom, and left margins.
In the example below, I set the bottom margin of dna1_plot to 0 and strip out all the bottom x-axis ticks and text. I also set the top margin of dna2_plot to 0. Doing this nearly makes the y-axis lines touch in the two plots.
dna1_plot <- ggplot(dna1, aes(x = conc, y = density)) + geom_point() + theme_classic() +
theme(axis.title.x = element_blank(),
axis.ticks.x = element_blank(),
axis.text.x = element_blank(),
plot.margin = unit(c(1,1,0,1), "mm"))
#Figure 2:
dna2_plot <- ggplot(dna2, aes(x = conc, y = density)) + geom_point() + theme_classic() +
theme(plot.margin = unit(c(0,1,1,1), "mm"))
(dna1_plot / dna2_plot)