scale_colour_manual() silently ignores drop = TRUE

[dholstius]

This might be expected behavior but it seems to conflict with the documentation:

Reprex

As Expected

library(tidyverse)

df <- mutate(mtcars, cyl = factor(cyl, levels = as.character(c(2,4,6,8))))

p <- ggplot(df, aes(mpg, wt, color = cyl)) + geom_point()
p + scale_colour_discrete(drop = TRUE)  # omits "2" from legend, as expected
p + scale_colour_discrete(drop = FALSE) # puts "2" in legend, as expected

Unexpected

# imagine this is a palette being used widely across an organization
my_global_palette <- c("2" = "black", "4" = "orange", "6" = "blue", "8" = "darkgreen") 

# puts "2" in legend, even though it's not in the levels that appear in the data
p + scale_colour_manual(values = my_global_palette, drop = TRUE) 

Description

?scale_manual says:

drop
Should unused factor levels be omitted from the scale? The default, TRUE, uses the levels that appear in the data; FALSE uses all the levels in the factor.

scale_colour_manual(..., drop = TRUE) silently accepts the drop argument, but in the last example above, the factor level "2" is still shown in the legend.

Maybe the intent is for values to take precedence over the drop argument, but (a) this is unclear from the documentation, and (b) I think a case could be made against it. A user may wish to define and reuse a default palette without all elements of that palette appearing every time. (Maybe that's achievable in some other way?)

[dholstius]

Looks like this may be resolved by #4619 — I'll be patient and use limits = force in the meantime.

Will drop = FALSE be honored in the future, even though the default will correspond to drop = TRUE?

[teunbrand]

As far as I can tell, it does what you expect in the development version of ggplot2.

# devtools::install_github("tidyverse/ggplot2")
library(tidyverse)

my_global_palette <- c("2" = "black", "4" = "orange", "6" = "blue", "8" = "darkgreen") 

df <- mutate(mtcars, cyl = factor(cyl, levels = as.character(c(2,4,6,8))))

p <- ggplot(df, aes(mpg, wt, color = cyl)) + geom_point()

p + scale_colour_manual(values = my_global_palette, drop = TRUE) 

p + scale_colour_manual(values = my_global_palette, drop = FALSE) 

^{Created on 2022-02-03 by the reprex package (v2.0.1)}

[yutannihilation]

Closing as this is already resolved by #4619. (If I'm missing something, please let me know)