Legend title is incorrectly aligned when horizontal

[hadley]

library(ggplot2)

df <- data.frame(z = 1:5)
p <- ggplot(df, aes(1, 1, colour = z)) + geom_point()

# Title should be centered with legend
p + theme(legend.position = "top")

[hadley]

@clauswilke have you seen this too? I quickly skimmed through your PRs but I didn't see anything that addressed this explicitly

[clauswilke]

I haven't addressed or modified this in any way. What is happening is that the title is centered relative to the entire legend:

p + theme(legend.position = "top") + 
  theme(legend.spacing.y = grid::unit(50, "cm")) # 50 cm equals to 5 cm because of issue #2398

[clauswilke]

Note that guide_legend() behaves exactly the same:

df <- data.frame(z = 1:5)
p <- ggplot(df, aes(1, 1, colour = factor(z))) + 
  geom_point() + 
  scale_color_discrete(guide = guide_legend(label.position = "bottom"))

# Title should be centered with legend
p + theme(legend.position = "top") + theme(legend.spacing.y = grid::unit(5, "cm"))

[hadley]

Oh so this probably isn't a regression

[clauswilke]

The bigger issue, in my mind, is that there is no simple workaround, because the vjust setting is ignored. I'm showing this here with both a discrete and a continuous scale, to show that they behave the same:

library(ggplot2)
p <- ggplot(mtcars, aes(hp, disp, color = mpg, shape = factor(cyl))) +
  geom_point(size = 2) +
  scale_shape_discrete(guide = guide_legend(label.position = "bottom")) + 
  theme(legend.position = "top")
p

p + theme(legend.title = element_text(vjust = 1, colour = "red"))

(I've colored the legend title red to show that styling the legend title works, only the justification arguments are ignored.)

I'll see if there's a simple way to fix this that doesn't require touching much code. In the long run, I think the legend drawing code needs to be rethought from the ground up, but that's not for 2.3.0.

[ptoche]

Workaround of sorts:

library(ggplot2)
ggplot(mtcars, aes(hp, disp, color = mpg, shape = factor(cyl))) +
    geom_point(size = 2) +
    theme(legend.position = "top") +
    guides(shape = guide_legend(label.position = "bottom", 
            title.position = "left", title.vjust = 0.8), 
        color = guide_legend(label.position = "top", 
            title.position = "right", title.vjust=0.4))

Created on 2018-05-07 by the reprex package (v0.2.0).

[clauswilke]

I now see that there are (at least) two separate issues. One is that vjust and hjust can't be set in the theme but can be set via the guide function. It is a simple fix to allow setting via the theme as well. The second is that the margin settings in legend.title are ignored. This is a problem because if we set vjust to a number other than 1 then the text is moved different amounts for grobs of different heights. I guess the workaround could be to manually fiddle with the title.vjust settings for each guide function until things look right, but if margins could be set that would be better I think.

library(ggplot2)
ggplot(mtcars, aes(hp, disp, color = mpg, shape = factor(cyl))) +
  geom_point(size = 2) + theme(legend.position = "top") +
  guides(color = guide_colorbar(title.vjust = .8),
         shape = guide_legend(title.vjust = .8)) +
  theme(legend.title = 
          element_text(
            debug = TRUE,
            margin = margin(10, 0, 0, 0, "pt") # margin setting is ignored
          )
       )

[karawoo]

The latter issue about the margins for legend text/titles has been raised in #1502. I tried to tackle that after reworking titleGrob() last year, but wasn't able to wrap my head around the legend code enough to make any headway at the time.

[clauswilke]

@karawoo I just opened a pull request (#2556) that provides a partial fix for this issue. I hadn't seen #1502 previously and will take a look. My fix for legend titles would likely work for legend text as well. (Meaning: we can use the same technique to fix legend text. But it isn't currently fixed in the pending pull request.)