Center-align legend title and legend keys in ggplot2 for long legend titles

Question

I am having a hard time making the title of a legend center-aligned relative to the legend keys when the legend title is long. There is a question from a year ago that works for short titles, but it doesn't seem to work for long ones.

Example, first with a short legend title:

library(ggplot2)
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 = "A") +
  theme(legend.title.align = 0.5)

Everything is as expected, the legend title is centered above the legend key.

Now the same with a long legend title:

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)

We can see that the text is center aligned to itself but not relative to the legend key. I have tried modifying other theme options, such as legend.justification = "center", but none seem to move the key from its left-most position in the legend box.

A couple of comments:

I'm running the development version of ggplot2, v2.2.1.9000 from a few days ago.
I specifically need a solution for a continuous colorscale palette.

score 21 · Answer 1 · answered Oct 01 '19 at 17:05

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.

Claus Wilke · Accepted Answer · 2019-10-04T05:38:37.000

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)

score 6 · Answer 3 · answered Dec 28 '17 at 00:56

6

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.

answered Dec 28 '17 at 00:56

baptiste

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Thanks, that's what I was worried about. I'll start by opening an issue. – Claus Wilke Dec 28 '17 at 01:16
Issues have their own issues though; if you overdo it there might be [unfortunate consequences](https://github.com/tidyverse/ggplot2/issues/816#issuecomment-35949082). – baptiste Dec 28 '17 at 01:28
Ah, I see, it needs to come with a pull request. Maybe for another day then, I don't immediately see how to fix this without adding yet another theme parameter. – Claus Wilke Dec 28 '17 at 01:44
Can you think of a simple way to edit the legend grob to work around this ggplot2 shortcomming? – Claus Wilke Dec 28 '17 at 03:18
the guides code makes very little sense to me and as far as I know doesn't come with much explanation, but [from what I _vaguely_ recall](https://github.com/tidyverse/ggplot2/issues/606) the general strategy for handling justification with gtable is to make it a child gtable and position it accordingly within its parent (then again, looking at the output guide grobs is already a russian doll of nested gtables, so adding one layer is maybe not such a great idea). – baptiste Dec 28 '17 at 03:45
It turns out it's not that hard after all. We can just add an empty column into the gtable. There even is an appropriate width lying around that we can reuse to get things perfectly centered. I posted this as an answer. – Claus Wilke Dec 28 '17 at 17:38
I think changing the source code will be necessary one way or another. It is incredibly buggy. After looking it over a little more carefully, I discovered at least 3 major bugs, including a crash (ggplot2 exits instead of drawing), a vertical unit being used in a horizontal context, and a unit being converted to cm and then used as mm. Looks like the code has not been properly maintained for a while. – Claus Wilke Dec 30 '17 at 18:32
i reported [some bugs along these lines](https://github.com/tidyverse/ggplot2/issues/1881) not long ago, and realised that guide_colourbar has a lot of code duplication and seems to have been written by someone else. As I said, my personal view is that the whole guides code should be re-written from scratch with a global well-defined and documented strategy. It's probably a hard thing to prioritise though, because it's always been kind of good enough, so the few user-facing glitches often sound like easy fixes until one peeks behind the curtains. – baptiste Dec 30 '17 at 22:36

Center-align legend title and legend keys in ggplot2 for long legend titles

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