Solid arrows with segments that are not solid have visual artifacts

[malcolmbarrett]

Someone reported an issue in ggdag with dashed arrows with solid arrowheads (r-causal/ggdag#216). I checked both ggplot2 and ggraph, and it seems like this may be an issue in ggplot2.

Creating a dashed line with a solid arrowhead results in visual artifacts. It looks like the dashed line might ALSO have an arrowhead behind it

library(ggplot2)
library(grid)

df <- data.frame(
  x = c(0, 0),
  y = c(0, 0.5),
  xend = c(1, 1),
  yend = c(0, 0.5),
  linetype = c("solid", "dashed")
)

ggplot(df) +
  geom_segment(
    aes(x, y, xend = xend, yend = yend, linetype = linetype),
    linewidth = 1,
    arrow = arrow(length = unit(0.3, "cm"), type = "closed")
  ) +
  coord_fixed(xlim = c(-0.1, 1.1), ylim = c(-0.2, 0.9)) +
  scale_linetype_identity() +
  theme_void()

^{Created on 2025-11-17 with reprex v2.1.1}

[teunbrand]

This is because the linetype is also applied to the lines that make up the arrow head.
The grid package is responsible for drawing these arrows, so there isn't really anything ggplot2 can do to control this.

library(ggplot2)
#> Warning: package 'ggplot2' was built under R version 4.5.2

ggplot() +
  annotate(
    x = 1:2, y = 1, geom = "line", 
    arrow = arrow(type = "closed", length = unit(2, "cm")), 
    arrow.fill = NA,
    linetype = 2
  )

^{Created on 2025-11-17 with reprex v2.1.1}

[malcolmbarrett]

thanks! addressing this seems like it would have to be a feature request to grid + modifications to ggplot2. Doesn't seem worth it unless it bothers others

[teunbrand]

Apologies for the self-advertisement, but my own frustrations with grid's arrows have led to ggarrow, which may or may not solve this exact issue.