bug(geom_bar(position="fill")): Single combination groups are not transformed to proportions

[schloerke]

I found a problem with geom_bar(position="fill")

I expected the plot's y axis to have a proportion where it ranges from 0 to 1. However, when the color combinations produce single values, the count is provided, not the proportion of 100%.

Here is the code to reproduce the bug:

library(ggplot2)

# Good: Y axis is proportion
ggplot(mtcars, aes(factor(cyl), fill = factor(vs))) + geom_bar(position="fill")

ggplot(GGally::tips, aes(x = smoker, fill = sex)) + geom_bar(position = "fill")

# Bad: Y axis is count, not proportion
ggplot(subset(mtcars, cyl == 8), aes(factor(cyl), fill = factor(vs))) + geom_bar(position="fill")

ggplot(GGally::tips, aes(x = smoker, fill = smoker)) + geom_bar(position = "fill")

^{Created on 2025-06-19 with reprex v2.1.1}

sessioninfo::package_info("ggplot2")
#>  ggplot2      * 3.5.2.9001 2025-06-19 [1] Github (tidyverse/ggplot2@a2d0181)

[schloerke]

Reprex using {ggplot2} v3.5.2 runs as expected:

library(ggplot2)

# Good: Y axis is proportion
ggplot(mtcars, aes(factor(cyl), fill = factor(vs))) + geom_bar(position="fill")

ggplot(GGally::tips, aes(x = smoker, fill = sex)) + geom_bar(position = "fill")
#> Registered S3 method overwritten by 'GGally':
#>   method from   
#>   +.gg   ggplot2

# ~~Bad: Y axis is count, not proportion~~
# Good!
ggplot(subset(mtcars, cyl == 8), aes(factor(cyl), fill = factor(vs))) + geom_bar(position="fill")

ggplot(GGally::tips, aes(x = smoker, fill = smoker)) + geom_bar(position = "fill")

^{Created on 2025-06-19 with reprex v2.1.1}

[teunbrand]

Thanks for the report! I'm glad you and @IndrajeetPatil caught this now, rather than after release.
It is a position_fill() problem specifically, which is also apparent with other geoms, e.g. #6520.