Add group mean line to barplot with ggplot2

With your existing ggplot, Try This Code:

+
geom_hline(data = [*name of data frame*], aes(yintercept = mean(*name of the variable*), color = "red")

I combined the solution from @bouncyball with my original approach using `geom_errorbar.

Here is the code:

df.mean = df %>% 
  group_by(class) %>% 
  mutate(ymean = mean(y))

ggplot(df, aes(x, y, fill=class)) +
  geom_col() +
  geom_errorbar(data=df.mean, aes(x, ymax = ymean, ymin = ymean),
               size=0.5, linetype = "longdash", inherit.aes = F, width = 1)

The only problem is that instead of single line this approach generate a lot of line objects which can be seen when editing the plot, for example, in Adobe Illustrator. But I can live with it.

UPDATE

Another solution - simpler and without the above problem. Again based on the code from @bouncyball.

df.mean = df %>% 
  group_by(class) %>% 
  summarise(ymean = mean(y), x1 = x[which.min(x)], x2 = x[which.max(x)]) %>% 
  ungroup()

ggplot(df) +
  geom_col(aes(x, y, fill = class)) +
  geom_segment(data = df.mean,
               aes(x = as.integer(x1) - 0.5, xend = as.integer(x2) + 0.5,
                   y = ymean, yend = ymean),
               size=1, linetype = "longdash", inherit.aes = F)

Create a new data.frame (adding a group mean) and do some manipulations on it (using top_n and cbind), then use those to supply the necessary aesthetics to geom_segment:

# add group mean
df_m <- df %>%
  group_by(class) %>%
  mutate(my = mean(y)) %>%
  arrange(class) # added from comment by @Yuk

# select top and bottom x for each class group
# use cbind to keep one row per group
df_m2 <- df_m %>%
  top_n(1, x) %>%
  cbind(top_n(df_m, -1, x))

ggplot(df) +
  geom_col(aes(x, y, fill=class))+
  geom_segment(data = df_m2,
               aes(x = x, xend = x1,
                   y = my, yend = my1,
                   group = class))