mean line in every facet_wrap

Question

ggplot(data = results, aes(x = inst, y = value, group = inst)) +
  geom_boxplot() +
  facet_wrap(~color) +
  #geom_line(data = mean, 
          #mapping = aes(x = inst, y = average, group = 1))
  theme_bw()

When I run the code above with the code line commented, it runs and gives the graph below but I want a joining mean lines on the boxplots based on its own color category for each group in facet wraps. Any ideas how can I do that?

Answer 1

Your code is generally correct (though you'll want to add color = color to the aes() specification in geom_line()), so I suspect your mean dataset isn't set up correctly. Do you have means grouped by both your x axis and faceting variable? Using ggplot2::mpg as an example:

library(dplyr)   # >= v1.1.0
library(ggplot2)

mean_dat <- summarize(mpg, average = mean(hwy), .by = c(cyl, drv))

ggplot(mpg, aes(factor(cyl), hwy)) +
  geom_boxplot() + 
  geom_line(
    data = mean_dat, 
    aes(y = average, group = 1, color = drv),
    linewidth = 1.5,
    show.legend = FALSE
  ) +
  facet_wrap(~drv) +
  theme_bw()

Alternatively, you could use stat = "summary" and not have to create a means dataframe at all:

ggplot(mpg, aes(factor(cyl), hwy)) +
  geom_boxplot() + 
  geom_line(
    aes(group = 1, color = drv),
    stat = "summary",
    linewidth = 1.5,
    show.legend = FALSE
  ) +
  facet_wrap(~drv) +
  theme_bw()
# same result as above