r - ggplot: remove NA factor level in legend

Question

How can I omit the NA level of a factor from a legend?

From the nycflights13 database, I created a new continuous variable called tot_delay, and then created a factor called delay_class with 4 levels. When I plot, I filter out NA values, but they still appear in the legend. Here's my code:

library(nycflights13); library(ggplot2)

flights$tot_delay = flights$dep_delay + flights$arr_delay
flights$delay_class <- cut(flights$tot_delay,                                   
                           c(min(flights$tot_delay, na.rm = TRUE), 0, 20 , 120,
                             max(flights$tot_delay, na.rm = TRUE)),   
                           labels = c("none", "short","medium","long"))     

filter(flights, !is.na(tot_delay)) %>% 
  ggplot() +
  geom_bar(mapping = aes(x = carrier, fill = delay_class), position = "fill")

Answer 1

The parent example isn't a good illustration of the problem (of course unexpected NA values should be tracked down and eliminated), but this is the top result on Google so it should be noted that there is a now an option in scale_XXX_XXX to prevent NA levels from displaying in the legend by setting na.translate = F. For example:

# default    
ggplot(data = data.frame(x = c(1,2,NA), y = c(1,1,NA), a = c("A","B",NA)),
           aes(x, y, colour = a)) + geom_point(size = 4)

# with na.translate = F    
ggplot(data = data.frame(x = c(1,2,NA), y = c(1,1,NA), a = c("A","B",NA)),
           aes(x, y, colour = a)) + geom_point(size = 4) + 
           scale_colour_discrete(na.translate = F)

This works in ggplot2 3.1.0.