Vertical space between legend keys changed after switching theme?

[tungttnguyen]

Hello,

I noticed there was difference in the vertical space between legend keys when switching between themes e.g. from theme_grey to theme_bw (see the reprex below). I wonder what was the cause of that?

Thanks!

### ggplot2 3.1.0.9000 (2019-03-06)
library(ggplot2)

theme_set(theme_grey(base_size = 14))
p1 <- ggplot(mtcars) +
  aes(fill = factor(cyl), x = cyl) +
  geom_bar() +
  theme(
    legend.key = element_rect(size = 6),
    legend.key.size = unit(1, "cm")
  )
print(p1)

theme_set(theme_bw(base_size = 14))
p2 <- ggplot(mtcars) +
  aes(fill = factor(cyl), x = cyl) +
  geom_bar() +
  theme(
    legend.key = element_rect(size = 6),
    legend.key.size = unit(1, "cm")
  )
print(p2)

[ptoche]

each theme defines/redefines various properties. You can see the defaults here

Possibly the reason for what you see is here: theme_bw is based on theme_gray. The following is defined by theme_gray:

legend.key = element_rect(fill = "grey95", colour = "white")

and redefined by theme_bw:

legend.key = element_rect(fill = "white", colour = NA),

That, I'm guessing, explains the extra white color between legend keys.

To test that, use theme_update() with the legend.key redefined and see if you can match the plots.

P.S. I haven't looked at the source code, but setting colour = NA possibly removes some padding. Looks like it would be innocuous to change that to colour = white and, if everyone agrees, the legend would look "better" with that extra white space in between keys. Or not?

[ptoche]

It's down to the difference between colour = "white" and colour = NA:

library(reprex)
library(ggplot2)
theme_set(theme_bw(base_size = 14))
theme_update(legend.key = element_rect(size = 6, fill = "white", colour = NA), legend.key.size = unit(1, "cm"))
ggplot(mtcars) +
    aes(fill = factor(cyl), x = cyl) +
    geom_bar() 

^{Created on 2019-03-15 by the reprex package (v0.2.1)}

library(reprex)
library(ggplot2)
theme_set(theme_bw(base_size = 14))
theme_update(legend.key = element_rect(size = 6, fill = "white", colour = "white"), legend.key.size = unit(1, "cm"))
ggplot(mtcars) +
    aes(fill = factor(cyl), x = cyl) +
    geom_bar() 

^{Created on 2019-03-15 by the reprex package (v0.2.1)}

[clauswilke]

Setting the color to black makes it clearer what happens. The spacing doesn't change, but there's an additional border that is drawn and that covers some of the key parts. Maybe we should remove this border for theme_bw()?

library(ggplot2)
theme_set(theme_bw(base_size = 14))
theme_update(legend.key = element_rect(size = 6, fill = "white", colour = "black"), legend.key.size = unit(1, "cm"))
ggplot(mtcars) +
  aes(fill = factor(cyl), x = cyl) +
  geom_bar() 

^{Created on 2019-03-15 by the reprex package (v0.2.1)}

[tungttnguyen]

Thanks @ptoche & @clauswilke ! That makes sense. So is there a way to make the size of those legend boxes the same? The blue key box is noticeably larger than the others.

[clauswilke]

I would recommend to just set colour = NA in the theme element. We should probably make that fix in the ggplot2 code base also.

library(ggplot2)
theme_set(theme_bw(base_size = 14))
theme_update(legend.key = element_rect(size = 6, fill = "white", colour = NA), legend.key.size = unit(1, "cm"))
ggplot(mtcars) +
  aes(fill = factor(cyl), x = cyl) +
  geom_bar() 

^{Created on 2019-03-15 by the reprex package (v0.2.1)}

[tungttnguyen]

Thanks @clauswilke ! Now I understand. Would be great if this is fixed int the future release

[yutannihilation]

@clauswilke

Maybe we should remove this border for theme_bw()?

I would recommend to just set colour = NA in the theme element. We should probably make that fix in the ggplot2 code base also.

Sorry, I'm confused... Are you talking about theme_gray()? theme_bw() doesn't have borders, and theme_gray() does.

library(ggplot2)

theme_bw()$legend.key$colour
#> [1] NA
theme_gray()$legend.key$colour
#> [1] "white"

^{Created on 2019-03-18 by the reprex package (v0.2.1.9000)}

[clauswilke]

I may have mixed them up. The borders seem to be causing trouble (uneven glyph size), so it may be better to remove them and to create spacing between the keys in a different manner.

[tungttnguyen]

@clauswilke & @yutannihilation : is there any reason why ggplot doesn't have an option to manipulate vertical distance between legend key boxes? Thanks!

[clauswilke]

There's no good principled reason. It's just that the way legend drawing is currently implemented, adding variable vertical spacing between legend key boxes will not be trivial to implement.

[ptoche]

Looking at it again, setting colour = NA looks like the correct thing to do: Had you noticed how the blue square in the legend appears taller than the other two squares? (I hadn't the first time around) It seems that the covering line (the black line in Claus's example) is not vertically centered, thus creating the undesired effect. Must be fixed.