Indexes withing ggplot in R

Question

I am generating some plots with the following code.I have 8 plots generated with the the following code and what I want is to have them on the same page with no titles. More specifically, I want in every plot to have on the left up-corner a letter (a,b..) and at the end of the plot to have something like an one-row legend (e.g Plots: a. category one, b. category two, ...).

Code:

g1= ggplot(som, aes(x=value, y=variable))+geom_smooth(method=lm,alpha=0.25,col='green',lwd=0.1) +ylim(0,1000)+xlim(-2,2)+
         geom_point(shape=23,fill="black",size=0.2)+theme_bw()+theme(plot.background = element_blank(),panel.grid.major = element_blank()
        ,panel.grid.minor = element_blank()) +labs(x="something here",y="something else")+
        theme(axis.title.x = element_text(face="bold", size=7),axis.text.x = element_text(size=5))+
        theme(axis.title.y = element_text(face="bold", size=7),axis.text.y = element_text(size=5))+
        theme(plot.title = element_text(lineheight=.8, face="bold",size=8))
grid.arrange(g1,g2,g3,g4,g5,g6,g7,g8,ncol=2)

Is it possible to do that with ggplot? If so, how can I do this?

p.s I have no problem with the above code

Thank you.

Answer 1

This is how you could do it with library(cowplot).

First some plots:

set.seed(1)
plots <- list()

for (i in 1:8) {
    my_cars <- mtcars[sample(1:nrow(mtcars), 10), ]

    plots[[i]] <- ggplot(my_cars, aes(mpg, hp, color = as.factor(cyl))) +
        geom_point() +
        geom_smooth(method = "lm", color = "black")
}

Then to have a unifying title (or legend here) we use a combination of two plot_grid() calls.

lbls <- LETTERS[1:length(plots)]
# add a line break because its long
lbls <- gsub("E", "\nE", lbls)

grid <- plot_grid(plotlist = plots, labels = lbls, ncol = 2)

legend <- ggdraw() +
    draw_label(paste0(lbls, "= category",1:length(plots), collapse = "  "))

plot_grid(grid, legend, rel_heights = c(1, .1), ncol = 1)

The documentation for cowplot is great and has a ton of examples. Check it out here and here. Let me know if you get stuck.