Shift legend into empty facets of a faceted plot in ggplot2
I think lemon::reposition_legend()
identified by @RLave is the most elegant solution. However, it does hinge on knowing the names of empty facets. I wanted to share a succinct way of finding these, thus proposing yet another version of shift_legend()
:
shift_legend3 <- function(p) {
pnls <- cowplot::plot_to_gtable(p) %>% gtable::gtable_filter("panel") %>%
with(setNames(grobs, layout$name)) %>% purrr::keep(~identical(.x,zeroGrob()))
if( length(pnls) == 0 ) stop( "No empty facets in the plot" )
lemon::reposition_legend( p, "center", panel=names(pnls) )
}
The R package patchwork
offers an elegant solution when combining multiple plots (slightly different than a single facetted ggplot). If one has three ggplot objects, p1, p2, p3, then the syntax is very straightforward:
- using the
+
operator, "add" the plots together in facets - using the
guide_area()
command, specify which facet should contain the guide - if all three plots have the same legend, save space by telling patchwork to "collect" the legends with the command
plot_layout(guides = 'collect')
.
See the code below for the essential syntax and the link below for a fully reproducible example.
library(patchwork)
# guide_area() puts legend in empty fourth facet
p1 + p2 + p3 + guide_area() +
plot_layout(guides = 'collect')
https://patchwork.data-imaginist.com/articles/guides/layout.html#controlling-guides
Nice Q&A!
I found something similar at this link. So, I thought that it would have been a nice addition to your function.
More precisely the function reposition_legend()
from lemon
seems to be quite what you needed, except that it doesn't look for the empty spaces.
I took inspiration from your function to find the names of the empty panels that are passed to reposition_legend()
with the panel
arg.
Example data and libraries:
library(ggplot2)
library(gtable)
library(lemon)
p <- ggplot(diamonds,
aes(x = carat, fill = cut)) +
geom_density(position = "stack") +
facet_wrap(~ color) +
theme(legend.direction = "horizontal")
Of course, I removed all the checks (if
cases, which should be the same) just to concentrate on the important stuff.
shift_legend2 <- function(p) {
# ...
# to grob
gp <- ggplotGrob(p)
facet.panels <- grep("^panel", gp[["layout"]][["name"]])
empty.facet.panels <- sapply(facet.panels, function(i) "zeroGrob" %in% class(gp[["grobs"]][[i]]))
empty.facet.panels <- facet.panels[empty.facet.panels]
# establish name of empty panels
empty.facet.panels <- gp[["layout"]][empty.facet.panels, ]
names <- empty.facet.panels$name
# example of names:
#[1] "panel-3-2" "panel-3-3"
# now we just need a simple call to reposition the legend
reposition_legend(p, 'center', panel=names)
}
shift_legend2(p)
Note that this might still need some tweaking, I just thought it was something worth to be shared.
At the moment the behaviour seems OK, and the function is a few lines shorter.
Other cases.
First example:
p1 <- ggplot(economics_long,
aes(date, value, color = variable)) +
geom_line() +
facet_wrap(~ variable,
scales = "free_y", nrow = 2,
strip.position = "bottom") +
theme(strip.background = element_blank(),
strip.placement = "outside")
shift_legend2(p1)
Second example:
p2 <- ggplot(mpg,
aes(x = displ, y = hwy, color = fl, shape = factor(cyl))) +
geom_point(size = 3) +
facet_wrap(~ class, dir = "v") +
theme(legend.box = "horizontal")
#[1] "panel-2-3" "panel-3-3" are the names of empty panels in this case
shift_legend2(p2)
Third example:
p3 <- ggplot(mtcars,
aes(x = factor(1), fill = factor(cyl))) +
geom_bar(width = 1, position = "fill") +
facet_wrap(~ gear, nrow = 2) +
coord_polar(theta = "y") +
theme_void()
shift_legend2(p3)
Complete function:
shift_legend2 <- function(p) {
# check if p is a valid object
if(!(inherits(p, "gtable"))){
if(inherits(p, "ggplot")){
gp <- ggplotGrob(p) # convert to grob
} else {
message("This is neither a ggplot object nor a grob generated from ggplotGrob. Returning original plot.")
return(p)
}
} else {
gp <- p
}
# check for unfilled facet panels
facet.panels <- grep("^panel", gp[["layout"]][["name"]])
empty.facet.panels <- sapply(facet.panels, function(i) "zeroGrob" %in% class(gp[["grobs"]][[i]]),
USE.NAMES = F)
empty.facet.panels <- facet.panels[empty.facet.panels]
if(length(empty.facet.panels) == 0){
message("There are no unfilled facet panels to shift legend into. Returning original plot.")
return(p)
}
# establish name of empty panels
empty.facet.panels <- gp[["layout"]][empty.facet.panels, ]
names <- empty.facet.panels$name
# return repositioned legend
reposition_legend(p, 'center', panel=names)
}
The following is an extension to an answer I wrote for a previous question about utilising the space from empty facet panels, but I think it's sufficiently different to warrant its own space.
Essentially, I wrote a function that takes a ggplot/grob object converted by ggplotGrob()
, converts it to grob if it isn't one, and digs into the underlying grobs to move the legend grob into the cells that correspond to the empty space.
Function:
library(gtable)
library(cowplot)
shift_legend <- function(p){
# check if p is a valid object
if(!"gtable" %in% class(p)){
if("ggplot" %in% class(p)){
gp <- ggplotGrob(p) # convert to grob
} else {
message("This is neither a ggplot object nor a grob generated from ggplotGrob. Returning original plot.")
return(p)
}
} else {
gp <- p
}
# check for unfilled facet panels
facet.panels <- grep("^panel", gp[["layout"]][["name"]])
empty.facet.panels <- sapply(facet.panels, function(i) "zeroGrob" %in% class(gp[["grobs"]][[i]]))
empty.facet.panels <- facet.panels[empty.facet.panels]
if(length(empty.facet.panels) == 0){
message("There are no unfilled facet panels to shift legend into. Returning original plot.")
return(p)
}
# establish extent of unfilled facet panels (including any axis cells in between)
empty.facet.panels <- gp[["layout"]][empty.facet.panels, ]
empty.facet.panels <- list(min(empty.facet.panels[["t"]]), min(empty.facet.panels[["l"]]),
max(empty.facet.panels[["b"]]), max(empty.facet.panels[["r"]]))
names(empty.facet.panels) <- c("t", "l", "b", "r")
# extract legend & copy over to location of unfilled facet panels
guide.grob <- which(gp[["layout"]][["name"]] == "guide-box")
if(length(guide.grob) == 0){
message("There is no legend present. Returning original plot.")
return(p)
}
gp <- gtable_add_grob(x = gp,
grobs = gp[["grobs"]][[guide.grob]],
t = empty.facet.panels[["t"]],
l = empty.facet.panels[["l"]],
b = empty.facet.panels[["b"]],
r = empty.facet.panels[["r"]],
name = "new-guide-box")
# squash the original guide box's row / column (whichever applicable)
# & empty its cell
guide.grob <- gp[["layout"]][guide.grob, ]
if(guide.grob[["l"]] == guide.grob[["r"]]){
gp <- gtable_squash_cols(gp, cols = guide.grob[["l"]])
}
if(guide.grob[["t"]] == guide.grob[["b"]]){
gp <- gtable_squash_rows(gp, rows = guide.grob[["t"]])
}
gp <- gtable_remove_grobs(gp, "guide-box")
return(gp)
}
Result:
library(grid)
grid.draw(shift_legend(p))
Nicer looking result if we take advantage of the empty space's direction to arrange the legend horizontally:
p.new <- p +
guides(fill = guide_legend(title.position = "top",
label.position = "bottom",
nrow = 1)) +
theme(legend.direction = "horizontal")
grid.draw(shift_legend(p.new))
Some other examples:
# example 1: 1 empty panel, 1 vertical legend
p1 <- ggplot(economics_long,
aes(date, value, color = variable)) +
geom_line() +
facet_wrap(~ variable,
scales = "free_y", nrow = 2,
strip.position = "bottom") +
theme(strip.background = element_blank(),
strip.placement = "outside")
grid.draw(shift_legend(p1))
# example 2: 2 empty panels (vertically aligned) & 2 vertical legends side by side
p2 <- ggplot(mpg,
aes(x = displ, y = hwy, color = fl, shape = factor(cyl))) +
geom_point(size = 3) +
facet_wrap(~ class, dir = "v") +
theme(legend.box = "horizontal")
grid.draw(shift_legend(p2))
# example 3: facets in polar coordinates
p3 <- ggplot(mtcars,
aes(x = factor(1), fill = factor(cyl))) +
geom_bar(width = 1, position = "fill") +
facet_wrap(~ gear, nrow = 2) +
coord_polar(theta = "y") +
theme_void()
grid.draw(shift_legend(p3))