I have two ggplot2::geom_pologons filled with a partially transparent colour.
I would like the key to display with the same alpha, but it does not.
The solution would ideally scale such that as I add additional polygons with the same fill and alpha colour the matching holds.
Example code:
chosen_alpha = 0.2
ggplot() +
theme_minimal() +
geom_polygon(
aes(
fill = "A",
x = c(1, 2, 2),
y = c(1, 1, 2)
),
alpha = chosen_alpha
) +
geom_polygon(
aes(
fill = "A",
x = c(3, 4, 4),
y = c(3, 3, 4)
),
alpha = chosen_alpha
) +
guides(
fill = guide_legend(
override.aes = list(
alpha = chosen_alpha
),
)
)
This does work if I only have one of the polygons, as follows.
chosen_alpha = 0.2
ggplot() +
theme_minimal() +
geom_polygon(
aes(
fill = "A",
x = c(1, 2, 2),
y = c(1, 1, 2)
),
alpha = chosen_alpha
) +
guides(
fill = guide_legend(
override.aes = list(
alpha = chosen_alpha
),
)
)
>Solution :
The issue is that for each geom a key glyph is drawn, i.e. you end up with multiple layers of slightly transparent rects which are drawn on top of each other and hence you end up with a darker color the more geoms are added.
The cleanest approach to fix your issue would be to use only one geom_polygon to draw your polygons. To this end put your data in a data.frame and account for the different polygons via the group aes.
library(ggplot2)
chosen_alpha <- 0.2
dat <- data.frame(
x = c(c(1, 2, 2), c(2, 3, 3)),
y = c(c(1, 1, 2), c(2, 2, 3)),
fill = "A",
group = rep(1:2, each = 3)
)
ggplot(dat) +
theme_minimal() +
geom_polygon(
aes(
fill = fill,
x = x,
y = y,
group = group
),
alpha = chosen_alpha
)
Second option would be to set the key_glyph to "blank" for all but one geom. This way the key glyph is only drawn once:
ggplot() +
theme_minimal() +
geom_polygon(
aes(
fill = "A",
x = c(1, 2, 2),
y = c(1, 1, 2)
),
alpha = chosen_alpha,
key_glyph = "blank"
) +
geom_polygon(
aes(
fill = "A",
x = c(2, 3, 3),
y = c(2, 2, 3)
),
alpha = chosen_alpha
)
