r - How to change color of facet borders when using facet_grid

Question

When using facet_grid, ggplot2 divides the major categories that make up the facet variables with a broader-than-usual white line. This serves most purposes well. Sometimes I want to more clearly show the divisions between these major categorizations and would like to shade the facet division with another color. Is there a way to do that? Thanks.

Answer 1

Although a year late, I found this to be an easy fix:

ggplot(mpg, aes(cty, hwy, color = factor(year)))+ 
  geom_point()+
  facet_grid(cyl ~ drv) +
  theme(panel.margin=unit(.05, "lines"),
        panel.border = element_rect(color = "black", fill = NA, size = 1), 
        strip.background = element_rect(color = "black", size = 1))

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UPDATE 2021-06-01

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As of ggplot2 3.3.3, the property panel.margin is deprecated, and we should use panel.spacing instead. Therefore, the code should be:

ggplot(mpg, aes(cty, hwy, color = factor(year)))+ 
  geom_point()+
  facet_grid(cyl ~ drv) +
  theme(panel.spacing = unit(.05, "lines"),
        panel.border = element_rect(color = "black", fill = NA, size = 1), 
        strip.background = element_rect(color = "black", size = 1))