Multiplot of multiplots in ggplot2 - r

Question

I recently discovered the multiplot function from the Rmisc package to produce stacked plots using ggplot2 plots/objects. What I am trying to do now is to create a multiplot of multiplots. Unfortunately, unlike the ggplot function, multiplot does not produce objects, so my issue cannot be resolved by simply nesting multiplot.
I will create a dataframe to make my point clear. In my dataframe named df, I have 3 columns: period, group and value. A certain value is recorded for each of 3 groups over 10 periods. (Note: I don't use a seed number below despite the use of the sample function because the focus is not numerical, it is graphical)
# Create a data frame for illustration purposes
df <- data.frame(period = rep(1:10, 3),
group = rep(LETTERS[1:3], each = 10),
value = sample(100, 30, replace = TRUE))
I then add a fourth column to df, which is the exponential transformation of the value column.
df$exp.value = exp(df$value)
I would like to create stacked plots allowing me to compare the values in each group to their exponential counterparts.
# Split dataframe by group
df_split <- split(df, df$group)
# Plots of values in each group
plots <- lapply(df_split, function(i){
ggplot(data = i, aes(x = period, y = value)) + geom_line()
})
# Plots of logged values in each group
plots_exp <- lapply(df_split, function(i){
ggplot(data = i, aes(x = period, y = exp.value)) + geom_line()
})
plots and plots_exp are both lists of 3 elements each containing ggplot objects. The first element of each list corresponds to group A, the second element corresponds to group B and the third element corresponds to group C.
In order to compare each group's values to the exponential values, I can use the multiplot function. Following is an example with group A:
multiplot(plots[[1]], plots_log[[1]], cols = 1)
How can I create a grid which will include the multiplot above as well as the ones for groups B and C? As if the code included ... + facet_grid(. ~ group)?

Answer 1

We can use cowplot package:
library(cowplot)
plot_grid(plots[[1]], plots_exp[[1]],
plots[[2]], plots_exp[[2]],
plots[[3]], plots_exp[[3]],
labels = c("A", "A", "B", "B", "C", "C"),
ncol = 1, align = "v")
We can output to a pdf looping through plots and plots_exp list objects. Every page will contain 2 plots. This is a better option when we have a lot of groups:
pdf("myPlots.pdf")
lapply(seq(length(plots)), function(i){
plot_grid(plots[[i]], plots_exp[[i]], ncol = 1, align = "v")
})
dev.off()
Another option is to prepare the data for ggplot and use facet as usual:
library(dplyr)
library(tidyr)
library(ggplot2)
gather(df, valueType, value, -c(group, period)) %>%
mutate(myGroup = paste(group, valueType)) %>%
ggplot(aes(period, value)) +
geom_line() +
facet_grid(myGroup ~ ., scales = "free_y")