Center legend name and legend keys in ggplot2 for long legendary headers

Question

It’s hard for me to make the title of the legend centered on the legend keys when the title of the legend is long. There is a question from a year ago that works for short titles, but doesn't seem to be suitable 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) 

As expected, the title of the legend is located above the legend key.

Now the same with the long name of the legend:

 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 see that the text is centered, but not relative to the legend key. I tried to change other theme options, such as legend.justification = "center" , but none of them seemed to move the key from the leftmost position in the legend field.

A couple of comments:

• I am using the ggplot2 v2.2.1.9000 development version released a few days ago.

• I especially need a solution for a continuous color palette.

Answer 1

October 4, 2019 update:

Some time ago, I wrote a fairly general function based on an original idea that I published here almost two years ago. This function is on github here , but it is 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 some examples of how this 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 have found a solution. This requires some digging in the coffin tree, and it may not work if there are many legends, but otherwise it seems like 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) 

Answer 2

I cracked the source code, similar to the one described by baptiste in one of the comments above: put the color bar / label / checkmark in the child gtable & position it so that it has the same range of rows / columns (depending on the direction of the legend) as the title.

This is still a hack, but I would like to think of it as a “hack once per session” approach, without having to repeat steps manually for each plot.

Demonstration with different title width / 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 several color legends:

 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") 

(Note: legend.box.just = "center" is required to properly align the two legends. I was worried for a while, since only the "top", "bottom", "left" and "right" are currently listed as valid parameter values but it turns out that both "centers" / "centers" are also accepted by the base grid::valid.just . I'm not sure why this is not explicitly mentioned in the help file ?theme , however, it works.)

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 } 

On 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 undo the change, do:

 untrace(ggplot2:::guide_gengrob.colorbar) 

Used version of the package: ggplot2 3.2.1.

Answer 3

you will have to change the source code. Currently it calculates the width of the grob header and the + labels, and left - aligns the panel + shortcuts in the viewport (gtable). It is hardcoded.