Unexpected behaviour of colors in geom_line() #6533
Description
This issue is related to coloring line segments individually when using geom_line() and scale_color_identity().
I' m drawing a simple ggplot:
library(ggplot2)
(d <- data.frame(x = 1:3, col = c("blue", "green", "red")))
#> x col
#> 1 1 blue
#> 2 2 green
#> 3 3 red
ggplot(d, aes(x, x, color = col, group = 1)) +
geom_line(lwd = 1) +
scale_color_identity()
Created on 2025-06-27 with reprex v2.1.1
This makes sense to me. Segments are colored using color of the starting point of the segments, and consequently the last color is ignored.
Now, I replace "red" by an NA:
d1 <- data.frame(x = 1:3, col = c("blue", "green", NA))
ggplot(d1, aes(x, x, color = col, group = 1)) +
geom_line(lwd = 1) +
scale_color_identity()I get the same plot (not shown), with a warning:
Warning message:
Removed 1 row containing missing values or values outside the scale range (`geom_line()`).
The warning may be considered unnecessary, but it's ok, since there is an NA the color column.
Now I duplicate the data and redraw, faceting by group:
In the right panel, the green segment is missing, which does not make any sense to me.
library(ggplot2)
d2 <- data.frame(x = 1:3, col = c("blue", "green", NA)) |>
tidyr::crossing(group = c("A", "B"))
d2
#> # A tibble: 6 × 3
#> x col group
#> <int> <chr> <chr>
#> 1 1 blue A
#> 2 1 blue B
#> 3 2 green A
#> 4 2 green B
#> 5 3 <NA> A
#> 6 3 <NA> B
ggplot(d2, aes(x, x, color = col, group = 1)) +
facet_wrap(~ group) +
geom_line(lwd = 2) +
scale_color_identity()
#> Warning: Removed 1 row containing missing values or values outside the scale range
#> (`geom_line()`).
Created on 2025-06-27 with reprex v2.1.1
NB: Probably, using geom_segment() may be the better choice here...
Session info:
sessioninfo::session_info()
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#> ──────────────────────────────────────────────────────────────────────────────Created on 2025-06-27 with reprex v2.1.1