Draw a box around a legend ggplot2

Question

I created a plot with a custom legend in ggplot2. I tried to draw a box around all the items in the legend, however I could only draw a box around each individual item. How can I create only one box around all the items?

library(ggplot2)

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),
        aspect.ratio = 1, axis.text = element_text(colour = 1, size = 12),
        legend.background = element_rect(linetype = 2, size = 0.5, colour = 1))

Answer 1

It seems that the legend.background rectangle overlaps the legend.box.background rectangle. An easy fix is to set legend.background = element_blank().

But then, in my opinion, the spacing in the legend is ugly. The legend titles take up too much space even with no title set. Fix this be setting legend.title = element_blank(). Also the spacing between the two legends is too large. Fix this by setting the space to zero legend.spacing.y = unit(0, "mm")

library(ggplot2)

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(legend.title = element_blank(),
        legend.spacing.y = unit(0, "mm"), 
        panel.border = element_rect(colour = "black", fill=NA),
        aspect.ratio = 1, axis.text = element_text(colour = 1, size = 12),
        legend.background = element_blank(),
        legend.box.background = element_rect(colour = "black"))