Retain grouping information for groups with missing values

[GregorDall]

I am coming here through an issue I encountered in ggpairs. I have missing data for one of my groups so some panels contain information about a different number of groups. I think this is a problem inherited from ggplot2, because it removes NA values from the data.

In this example I have colors for three groups.

data(iris)
ggplot(iris, aes(Petal.Length, fill = Species))  + geom_histogram()

If I remove data for one group, the colors are chosen differently, but I would like to retain the grouping information and therefore the colors provided, without manually setting colors using scale_fill_manual(). It also throws a warning. Is there a parameter which I could set that retains the groups with missing values (e.g. drop=FALSE)?

iris[iris$Species=="setosa","Petal.Length"] <- NA
ggplot(iris, aes(Petal.Length, fill = Species))  + geom_histogram()

Warning message:
Removed 50 rows containing non-finite values (stat_bin).

This is the ggpairs report:
ggobi/ggally#445 (comment)

[yutannihilation]

drop=FALSE

Yes, you can specify it in the fill scale.

library(ggplot2)

iris[iris$Species=="setosa","Petal.Length"] <- NA
ggplot(iris, aes(Petal.Length, fill = Species)) +
  geom_histogram() +
  scale_fill_discrete(drop = FALSE)
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.
#> Warning: Removed 50 rows containing non-finite values (`stat_bin()`).

^{Created on 2022-07-14 by the reprex package (v2.0.1)}

You can also use factor to avoid warnings.

library(ggplot2)

iris$Species <- factor(iris$Species)

iris <- iris[iris$Species != "setosa", ]

ggplot(iris, aes(Petal.Length, fill = Species)) +
  geom_histogram() +
  scale_fill_discrete(drop = FALSE)
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

^{Created on 2022-07-14 by the reprex package (v2.0.1)}

[GregorDall]

Thank you @yutannihilation