Layout of plots with a unique legend using ggplot

Question

I was trying to create a layout with plots sharing the same legend. The legend is on the top of the first plot, however, the next plot has a different scale. How can I solve this?

library(ggplot2)
library(gridExtra)

grid.arrange(

ggplot(mpg, aes(displ, cty)) + 
  geom_point(aes(shape = "Data")) +
  stat_smooth(aes(linetype = "Regression"), method = "lm", 
              formula = y ~ x, se = FALSE, colour = 1, size = 0.5) +
  scale_shape_manual(values = 1) +
  labs(shape = "", linetype = "") +
  theme_classic() + 
  theme(panel.border = element_rect(colour = "black", fill=NA, size = 0.5),
        aspect.ratio = 1, axis.text = element_text(colour = 1, size = 10),
        legend.position = "top")
,
ggplot(mpg, aes(displ, cty)) + 
  geom_point(shape = 1) +
  stat_smooth(method = "lm", 
              formula = y ~ x, se = FALSE, colour = 1, size = 0.5) +
  theme_classic() + 
  theme(panel.border = element_rect(colour = "black", fill=NA, size = 0.5),
        aspect.ratio = 1, axis.text = element_text(colour = 1, size = 10))
)

Answer 1

I don't know how to use grid.arrange, but here's a solution using my cowplot package. The idea is to separate the legend out from the plot and then put the three elements into one column. A similar approach would work with grid.arrange, I assume.

library(cowplot)

p1 <- ggplot(mpg, aes(displ, cty)) + 
  geom_point(aes(shape = "Data")) +
  stat_smooth(aes(linetype = "Regression"), method = "lm", 
              formula = y ~ x, se = FALSE, colour = 1, size = 0.5) +
  scale_shape_manual(values = 1) +
  labs(shape = "", linetype = "") +
  theme_classic() + 
  theme(panel.border = element_rect(colour = "black", fill=NA, size = 0.5),
        aspect.ratio = 1, axis.text = element_text(colour = 1, size = 10),
        legend.position = "top")

p2 <- ggplot(mpg, aes(displ, cty)) + 
  geom_point(shape = 1) +
  stat_smooth(method = "lm", 
              formula = y ~ x, se = FALSE, colour = 1, size = 0.5) +
  theme_classic() + 
  theme(panel.border = element_rect(colour = "black", fill=NA, size = 0.5),
        aspect.ratio = 1, axis.text = element_text(colour = 1, size = 10))

legend <- get_legend(p1)
plot_grid(legend, p1 + theme(legend.position = "none"), p2,
          ncol=1, rel_heights = c(0.1, 1, 1))