plot_grid(p1, p2, p3, p4, align='vh', vjust=1, scale = 1)

library(ggplot2)
library(cowplot)
library(grid)
library(gridExtra)

ToothGrowth$dose <- as.factor(ToothGrowth$dose)

#make 4 plots

p1<-ggplot(ToothGrowth, aes(x=dose, y=len)) +
  geom_boxplot()


p2<-ggplot(ToothGrowth, aes(x=dose, y=supp)) +
  geom_boxplot()

p3<-ggplot(ToothGrowth, aes(x=supp, y=len)) +
  geom_boxplot()

p4<-ggplot(ToothGrowth, aes(x=supp, y=dose)) +
  geom_boxplot()

#combine using cowplot

plot<-plot_grid(p1, p2, p3, p4, align='vh', vjust=1, scale = 1)

#create common x and y labels

y.grob <- textGrob("Common Y",
                   gp=gpar(fontface="bold", col="blue", fontsize=15), rot=90)

x.grob <- textGrob("Common X",
                   gp=gpar(fontface="bold", col="blue", fontsize=15))

#add to plot

grid.arrange(arrangeGrob(plot, left = y.grob, bottom = x.grob))