Version 4.0.0 issue with facet_grid & facet_wrap randomly drops levels from fill aes

[kjmtaylor22]

I found a problem with version 4.0.0 that is corrected only by returning to version 3.5.2 and may have something to do with the architecture migration. In short, a geom_area plot (with fill) that does not use faceting will show all fill levels, but once the faceting (both facet_grid and facet_wrap) is added, random levels are dropped from the plot but not from the legend. In this case, the last group facet (with containing just 1 subgroup level) does not fill.

I am using R v4.5.1 (RStudio v2024.12.1.563) on Ubuntu 22.04.

Here is example code that reproduces the bug on my machine in ggplot2 v4.0.0:

df <- data.frame(
x=c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2),
y=c(74.04, 30.63, 17, 9.42, 3.12, 9.75, 2.54, 1.4, 1.28, 0.25, 0.22, 22.05, 10.5, 9.45, 7.41, 1.69, 24.6, 11, 70, 57.78, 23.91, 17, 7.73, 2.53, 9.75, 2.54, 1.4, 1.28, 0.25, 0.22, 14.7, 10.5, 9.45, 4.94, 1.44, 25.75, 11.55, 70),
group=rep(rep(LETTERS[1:5],times=c(5,6,5,2,1)), 2),
subgroup=rep(letters[1:19],2),
col=rep(c('#6A0215','#9A031E','#FA0F3A','#FB4062','#FC738C','#5A2B0D','#813B0F','#A84B11','#CF5B13','#E87B36','#EC8A4D','#104755','#1E8099','#2CB9DD','#5AC8E4','#8AD8EC','#666666','#CCCCCC','#0B3B0B'),2)
)

## Without faceting
ggplot(df, aes(x, y, fill=subgroup)) +
  scale_fill_manual(breaks=df$subgroup, values=df$col) + 
  geom_area(position="stack", color='white', linewidth=0.1)

## With faceting
ggplot(df, aes(x, y, fill=subgroup)) +
  facet_wrap(vars(group), ncol=1, scales="free_y",  
             strip.position = "right", as.table = T) +
  scale_fill_manual(breaks=df$subgroup, values=df$col) + 
  geom_area(position="stack", color='white', linewidth=0.1)

Using the same code in v3.5.2, both plots run just fine. I will be reverting to v3.5.2 in the meantime.

[teunbrand]

Thanks for the report!

To simplify and show the reprex; this is as you would expect:

library(ggplot2)

df <- data.frame(
  x = c(1, 2, 1, 2, 1, 2),
  y = c(1, 1, 1, 1, 1, 1),
  g = c("A", "A", "B", "B", "C", "C"),
  f = c("x", "x", "x", "x", "y", "y")
)

ggplot(df, aes(x, y, fill = g)) +
  geom_area(position = "stack")

However, when one group is alone in a panel, it disappears (group C)

last_plot() +
  facet_wrap(vars(f))

From a brief messing around, it seems that stat = "align" may be the culprit, because the identity stat does not cause this.
This is also expected:

ggplot(df, aes(x, y, fill = g)) +
  geom_area(position = "stack", stat = "identity") +
  facet_wrap(vars(f))

^{Created on 2025-10-03 with reprex v2.1.1}