saving each modified facet in ggplot2 - r

Question

I try to save each Species data in iris data set to .png file using for loop. But before, that I would like to modify facet strip thickness as I needed to do in my real data plotting process.
However, when I attempted to write each facet
the following code below it just giving me the empty plots for each of these Species.
Here is my attempt,
library(ggplot2)
plot_list = list()
for (i in unique(iris$Species)) {
p = ggplot(iris[iris$Species == i, ], aes(x=Sepal.Length, y=Sepal.Width)) +
geom_point(size=3, aes(colour=Species))+
facet_wrap(~Species)
#this part to modify facet_wrap strips
g1 = ggplotGrob(p)
pos = c(unique(subset(g1$layout, grepl("panel", g1$layout$name), select = t)))
for(i in pos) g1$heights[i-1] = unit(0.4,"cm")
grobs = which(grepl("strip", g1$layout$name))
for(i in grobs) g1$grobs[[i]]$heights <- unit(1, "npc")
grid.newpage()
grid.draw(g1)
plot_list[[i]] = g1
}
#finally write the modified graphs to file
for (i in 1:3) {
file_name = paste("iris_plot_", i, ".png", sep="")
tiff(file_name)
print(plot_list[[i]])
dev.off()
}
Currently this code is producing the empty graphs and do not know why! Any help will be appreciated!

Answer 1

You don't need to modify strip height using ggplotGrob. Setting the relevant parameter in ggplot's theme() would do:
p1 = ggplot(iris[iris$Species == "setosa",],
aes(x = Sepal.Length, y = Sepal.Width)) +
geom_point() +
facet_wrap(~Species)
p2 = p1 + theme(strip.text.x = element_text(margin = margin(t = 10, b = 10)))
# note: default margin for top & bottom is 5.5
gridExtra::grid.arrange(p1, p2, ncol = 2)
As for the rest, you may wish to check the length of plot_list after the first loop. You initially assigned i to take on the unique values of iris$Species, then tried to use it as index for the list of plots. The first three elements of plot_list did not contain plots.
The following would work in this example. You probably need to make some modifications for the actual use case:
plot_list = list()
loop.list <- unique(iris$Species)
for (i in seq_along(loop.list)) {
p = ggplot(iris[iris$Species == loop.list[i], ],
aes(x = Sepal.Length, y=Sepal.Width)) +
geom_point(size = 3, aes(colour = Species))+
facet_wrap(~Species) +
theme(strip.text.x = element_text(margin = margin(t = 11, b = 11)))
plot_list[[i]] <- ggplotGrob(p)
}
for (i in 1:3) {
file_name = paste("iris_plot_", i, ".png", sep="")
tiff(file_name)
grid.draw(plot_list[[i]])
dev.off()
}