Center-align legend title and legend keys in ggplot2 for long legend titles
Update Oct. 4, 2019:
A while back I wrote a fairly general function based on the original idea I posted here almost two years ago. The function is on github here but it's not part of any officially published package. It is defined as follows:
align_legend <- function(p, hjust = 0.5)
{
# extract legend
g <- cowplot::plot_to_gtable(p)
grobs <- g$grobs
legend_index <- which(sapply(grobs, function(x) x$name) == "guide-box")
legend <- grobs[[legend_index]]
# extract guides table
guides_index <- which(sapply(legend$grobs, function(x) x$name) == "layout")
# there can be multiple guides within one legend box
for (gi in guides_index) {
guides <- legend$grobs[[gi]]
# add extra column for spacing
# guides$width[5] is the extra spacing from the end of the legend text
# to the end of the legend title. If we instead distribute it by `hjust:(1-hjust)` on
# both sides, we get an aligned legend
spacing <- guides$width[5]
guides <- gtable::gtable_add_cols(guides, hjust*spacing, 1)
guides$widths[6] <- (1-hjust)*spacing
title_index <- guides$layout$name == "title"
guides$layout$l[title_index] <- 2
# reconstruct guides and write back
legend$grobs[[gi]] <- guides
}
# reconstruct legend and write back
g$grobs[[legend_index]] <- legend
g
}
The function is quite flexible and general. Here are a few examples of how it can be used:
library(ggplot2)
library(cowplot)
#>
#> ********************************************************
#> Note: As of version 1.0.0, cowplot does not change the
#> default ggplot2 theme anymore. To recover the previous
#> behavior, execute:
#> theme_set(theme_cowplot())
#> ********************************************************
library(colorspace)
# single legend
p <- ggplot(iris, aes(Sepal.Width, Sepal.Length, color = Petal.Width)) + geom_point()
ggdraw(align_legend(p)) # centered
ggdraw(align_legend(p, hjust = 1)) # right aligned
# multiple legends
p2 <- ggplot(mtcars, aes(disp, mpg, fill = hp, shape = factor(cyl), size = wt)) +
geom_point(color = "white") +
scale_shape_manual(values = c(23, 24, 21), name = "cylinders") +
scale_fill_continuous_sequential(palette = "Emrld", name = "power (hp)", breaks = c(100, 200, 300)) +
xlab("displacement (cu. in.)") +
ylab("fuel efficiency (mpg)") +
guides(
shape = guide_legend(override.aes = list(size = 4, fill = "#329D84")),
size = guide_legend(
override.aes = list(shape = 21, fill = "#329D84"),
title = "weight (1000 lbs)")
) +
theme_half_open() + background_grid()
# works but maybe not the expected result
ggdraw(align_legend(p2))
# more sensible layout
ggdraw(align_legend(p2 + theme(legend.position = "top", legend.direction = "vertical")))
Created on 2019-10-04 by the reprex package (v0.3.0)
Original answer:
I found a solution. It requires some digging into the grob tree, and it may not work if there are multiple legends, but otherwise this seems a reasonable solution until something better comes along.
library(ggplot2)
library(gtable)
library(grid)
p <- ggplot(iris, aes(x=Sepal.Length, y=Sepal.Width, color=Petal.Width)) +
geom_point(size = 3) +
scale_color_distiller(palette = "YlGn", type = "seq", direction = -1,
name = "Long legend heading\nShould be centered") +
theme(legend.title.align = 0.5)
# extract legend
g <- ggplotGrob(p)
grobs <- g$grobs
legend_index <- which(sapply(grobs, function(x) x$name) == "guide-box")
legend <- grobs[[legend_index]]
# extract guides table
guides_index <- which(sapply(legend$grobs, function(x) x$name) == "layout")
guides <- legend$grobs[[guides_index]]
# add extra column for spacing
# guides$width[5] is the extra spacing from the end of the legend text
# to the end of the legend title. If we instead distribute it 50:50 on
# both sides, we get a centered legend
guides <- gtable_add_cols(guides, 0.5*guides$width[5], 1)
guides$widths[6] <- guides$widths[2]
title_index <- guides$layout$name == "title"
guides$layout$l[title_index] <- 2
# reconstruct legend and write back
legend$grobs[[guides_index]] <- guides
g$grobs[[legend_index]] <- legend
grid.newpage()
grid.draw(g)
you'd have to change the source code. Currently it computes the widths for the title grob and the bar+labels, and left-justifies the bar+labels in the viewport (gtable). This is hard-coded.
I hacked the source code similar to the way described by baptiste in one of the above comments: put the colour bar / label / ticks grobs into a child gtable, & position it to have the same row span / column span (depending on the legend's direction) as the title.
It's still a hack, but I'd like to think of it as a 'hack once for the whole session' approach, without having to repeat the steps manually for every plot.
Demonstration with different title widths / title positions / legend directions:
plot.demo <- function(title.width = 20,
title.position = "top",
legend.direction = "vertical"){
ggplot(iris,
aes(x=Sepal.Length, y=Sepal.Width, color=Petal.Width)) +
geom_point(size = 3) +
scale_color_distiller(palette = "YlGn",
name = stringr::str_wrap("Long legend heading should be centered",
width = title.width),
guide = guide_colourbar(title.position = title.position),
direction = -1) +
theme(legend.title.align = 0.5,
legend.direction = legend.direction)
}
cowplot::plot_grid(plot.demo(),
plot.demo(title.position = "left"),
plot.demo(title.position = "bottom"),
plot.demo(title.width = 10, title.position = "right"),
plot.demo(title.width = 50, legend.direction = "horizontal"),
plot.demo(title.width = 10, legend.direction = "horizontal"),
ncol = 2)
This works with multiple colourbar legends as well:
ggplot(iris,
aes(x=Sepal.Length, y=Sepal.Width,
color=Petal.Width, fill = Petal.Width)) +
geom_point(size = 3, shape = 21) +
scale_color_distiller(palette = "YlGn",
name = stringr::str_wrap("Long legend heading should be centered",
width = 20),
guide = guide_colourbar(title.position = "top"),
direction = -1) +
scale_fill_distiller(palette = "RdYlBu",
name = stringr::str_wrap("A different heading of different length",
width = 40),
direction = 1) +
theme(legend.title.align = 0.5,
legend.direction = "vertical",
legend.box.just = "center")
(Side note: legend.box.just = "center" is required to align the two legends properly. I was worried for a while since only "top", "bottom", "left", and "right" are currently listed as acceptable parameter values, but it turns out both "center" / "centre" are accepted as well, by the underlying grid::valid.just. I'm not sure why this isn't mentioned explicitly in the ?theme help file; nonetheless, it does work.)
To change the source code, run:
trace(ggplot2:::guide_gengrob.colorbar, edit = TRUE)
And change the last section of code from this:
gt <- gtable(widths = unit(widths, "cm"), heights = unit(heights,
"cm"))
... # omitted
gt
}
To this:
# create legend gtable & add background / legend title grobs as before (this part is unchanged)
gt <- gtable(widths = unit(widths, "cm"), heights = unit(heights, "cm"))
gt <- gtable_add_grob(gt, grob.background, name = "background",
clip = "off", t = 1, r = -1, b = -1, l = 1)
gt <- gtable_add_grob(gt, justify_grobs(grob.title, hjust = title.hjust,
vjust = title.vjust, int_angle = title.theme$angle,
debug = title.theme$debug), name = "title", clip = "off",
t = 1 + min(vps$title.row), r = 1 + max(vps$title.col),
b = 1 + max(vps$title.row), l = 1 + min(vps$title.col))
# create child gtable, using the same widths / heights as the original legend gtable
gt2 <- gtable(widths = unit(widths[1 + seq.int(min(range(vps$bar.col, vps$label.col)),
max(range(vps$bar.col, vps$label.col)))], "cm"),
heights = unit(heights[1 + seq.int(min(range(vps$bar.row, vps$label.row)),
max(range(vps$bar.row, vps$label.row)))], "cm"))
# shift cell positions to start from 1
vps2 <- vps[c("bar.row", "bar.col", "label.row", "label.col")]
vps2[c("bar.row", "label.row")] <- lapply(vps2[c("bar.row", "label.row")],
function(x) x - min(unlist(vps2[c("bar.row", "label.row")])) + 1)
vps2[c("bar.col", "label.col")] <- lapply(vps2[c("bar.col", "label.col")],
function(x) x - min(unlist(vps2[c("bar.col", "label.col")])) + 1)
# add bar / ticks / labels grobs to child gtable
gt2 <- gtable_add_grob(gt2, grob.bar, name = "bar", clip = "off",
t = min(vps2$bar.row), r = max(vps2$bar.col),
b = max(vps2$bar.row), l = min(vps2$bar.col))
gt2 <- gtable_add_grob(gt2, grob.ticks, name = "ticks", clip = "off",
t = min(vps2$bar.row), r = max(vps2$bar.col),
b = max(vps2$bar.row), l = min(vps2$bar.col))
gt2 <- gtable_add_grob(gt2, grob.label, name = "label", clip = "off",
t = min(vps2$label.row), r = max(vps2$label.col),
b = max(vps2$label.row), l = min(vps2$label.col))
# add child gtable back to original legend gtable, taking tlrb reference from the
# rowspan / colspan of the title grob if title grob spans multiple rows / columns.
gt <- gtable_add_grob(gt, justify_grobs(gt2, hjust = title.hjust,
vjust = title.vjust),
name = "bar.ticks.label", clip = "off",
t = 1 + ifelse(length(vps$title.row) == 1,
min(vps$bar.row, vps$label.row),
min(vps$title.row)),
b = 1 + ifelse(length(vps$title.row) == 1,
max(vps$bar.row, vps$label.row),
max(vps$title.row)),
r = 1 + ifelse(length(vps$title.col) == 1,
min(vps$bar.col, vps$label.col),
max(vps$title.col)),
l = 1 + ifelse(length(vps$title.col) == 1,
max(vps$bar.col, vps$label.col),
min(vps$title.col)))
gt
}
To reverse the change, run:
untrace(ggplot2:::guide_gengrob.colorbar)
Package version used: ggplot2 3.2.1.