Slight point strokes in ggplot points

Question

Consider the following:

library(ggplot2)

df = data.frame(x = rep(0,9), y = rep(0,9), alp = c(1:8/20,1))
ggplot(df) + 
  geom_point(aes(x, y, alpha=alp), size = 20, col = 'red') + 
  theme_minimal() + facet_wrap(~ alp) + guides(alpha = F)

As you can see there are feint outlines. It makes overlaying many low-transparency points look a bit like frogspawn. Is this just a Mac thing? Any idea how to remove it?

Are you talking about the very faint border of the point itself or are you talking about the underline of the background underneath the semi-transparent point? — Nick Becker, Jun 28 '16 at 20:23

Sandy Muspratt · Accepted Answer · 2016-12-07T05:42:13.270

The default point shape for ggplot2 is pch = 19. It's not one of those points where the colour of its border and its inside can be controlled separately; for instance, in the following, fill = 'black' has no effect.

library(ggplot2)

df = data.frame(x =runif(1000), y = runif(1000))

p = ggplot(df) + 
geom_point(aes(x, y), alpha = .1, size = 5, fill = 'black', colour = 'red') +                                        
  theme_bw() 
p

Yet the point does have a boundary line. The line's width can be changed with stroke; as follows:

p = ggplot(df) + 
geom_point(aes(x, y), stroke = 2, alpha = .1, size = 5, fill = 'black', colour = 'red') +                                        
  theme_bw() 
p

Unfortunately, setting stroke to zero will not remove the boundary line; it seems there is a lower limit.

To remove the boundary line, use one of the shapes that has a border that can be manipulated; for instance, shape = 21. Set its "fill" to red and its "colour" to transparent.

p = ggplot(df) + 
geom_point(aes(x, y), shape = 21, alpha = .1, size = 5, fill = 'red', colour = 'transparent') +                                      
  theme_bw() 
p

Whether the boundary vanishes when `stroke` is set to zero also depends on how you print the plot object: for example, when using `grDevices::svg()` to output an `svg`, the boundary disappears with `stroke = 0`. (I checked that with chromium; with SVG's appearance might also depend on your viewer.) — jarauh, Jan 23 '18 at 11:55
Also, if yo use `grDevices::png()` with `type = "cairo"`, the boundary disappears. — jarauh, Jan 23 '18 at 11:57

score 1 · Answer 2 · answered May 14 '20 at 22:27

see::geom_point2 draws points without this border.

library(ggplot2)
library(see)

df = data.frame(x = rep(0,9), y = rep(0,9), alp = c(1:8/20,1))

ggplot(df) + 
  geom_point2(aes(x, y, alpha=alp), size = 20, col = 'red') + 
  theme_minimal() + facet_wrap(~ alp) + guides(alpha = F)

^{Created on 2020-05-14 by the reprex package (v0.3.0)}

Slight point strokes in ggplot points

2 Answers2

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