Shift legend into empty facets of a faceted plot in ggplot2

Question

Consider the following plot:

library(ggplot2) p <- ggplot(diamonds, aes(x = carat, fill = cut)) + geom_density(position = "stack") + facet_wrap(~ color)

The facet_wrap function wraps a sequence of faceted panels into a roughly rectangular display of nrow rows and ncol columns. However, depending on the data, the actual number of panels is often a few panels short of nrow * ncol, which leaves a chunk of wasted space in the plot.

If the plot includes legend(s), the situation is exacerbated, because now we have even more wasted space due to the legend, whether it's on the right (default legend position), or one of the other three directions.

To save space, I would like to shift the legend(s) into the space created by unfilled facets.

The following works as a space-saving measure, but the legend is anchored to a corner of the plot area, with potentially a lot of space left on one side, creating an imbalanced look:

p + theme(legend.position = c(1, 0), legend.justification = c(1, 0))

Shifting a legend towards the centre of the blank space area by manually adjusting the legend.position/legend.justification values is a matter of trial and error, and difficult to scale if one has many faceted plots to work on.

In summary, I want a method that:

Shifts the legend(s) of a faceted plot into the space created due to empty facets.
Results in a reasonably nice-looking plot.
Is easily automated to handle many plots.

This is a recurring use case for me, and I've decided to post it along with my working solution here in case anyone else finds it useful. I haven't seen this scenario asked/answered elsewhere on Stack Overflow. If anyone has, please leave a comment and I'll be happy to answer there instead or have this marked as a duplicate, as the case may be.

double-beep · Accepted Answer · 2019-01-30 20:05:51Z

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))

Very nice solution! Your inheritance tests (!"gtable" %in% class(p) etc) should be written via inherits: if (! inherits(p, 'gtable')). And since you do different things depending on the object class, consider using S3 methods instead of if statements.
Nice answer. So is the shift.legend function not in a package?
Is there a way to convert the output back to a ggplot object?
A few changes are now required to make this work, you need change the 2 instances of guide-box to one of the following values: guide-box-right, guide-box-left, guide-box-bottom, guide-box-top, or guide-box-inside, depending on where your legend is currently.. See stackoverflow.com/questions/78163631/…

RLave · Accepted Answer · 2019-02-01 08:31:53Z

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) }

Is there a way to call the function without replotting the figure?
If I understand correctly your question you can modify the last line with reposition_legend(p, 'center', panel=names, plot = F) . Then you can do grid::grid.draw(shift_legend2(p3))
Thanks, that worked! I asked because my figures are complex and it takes time to plot them in Rstudio.

Artem Sokolov · Accepted Answer · 2019-11-06 16:49:54Z

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) ) }

Just found this function. I found this to be a superior solution and upvoted accordingly. Thank you.

Omar Wasow · Accepted Answer · 2021-08-14 11:12:36Z

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

Davor Josipovic · Accepted Answer · 2025-08-11 17:26:15Z

All existings answers are based on converting the initial plot to a grob, and then adjusting it before drawing. I wanted to achieve this without conversion to grobs, but directy on the ggplot object. Idea is to specify the location ggplot2::theme(legend.position.inside = ...) during plot definition based on default facet_wrap logic.

Number of facets can be easily derived from the data: nrow(unique(model.frame(facet_formula, data)))

The dimension of ggplot panel layout matrix based on the number of facets can be taken from ggplot2:::wrap_dims().

Then it is simply a calculation: determine where the empty facets start in the layout matrix and place the legend's top left point at exactly that spot. The function ltop.facetted() computes that location:

#' Left-top coordinate of first empty panel from plot perspective ltop.facetted <- function(nfacets, slot = "first_empty") { dm <- ggplot2:::wrap_dims(nfacets) nrow <- dm[1] ncol <- dm[2] # If nfacets is larger than total panels in grid, return NA. if (nfacets >= ncol * nrow) return(c(NA_real_, NA_real_)) # Compute the row/col index of the first empty facet slot # Fill order for facet_wrap is left-to-right, top-to-bottom empty.index <- nfacets + 1 empty.row <- ceiling(empty.index / ncol) empty.col <- empty.index - (empty.row - 1) * ncol # Convert to relative coordinates: (0,0) bottom-left to (1,1) top-right # Each cell width and height in relative coordinates: cell.width <- 1 / ncol cell.height <- 1 / nrow # Top-left corner of the empty slot: x <- (empty.col - 1) * cell.width y <- 1 - (empty.row - 1) * cell.height # This result can go into ggplot2::theme(legend.position.inside) together with # other arguments: legend.position = "inside" and legend.justification = c(0,1) structure(c(x, y), "nfree" = ncol * nrow - nfacets) }

Then simply plot like this:

ltop <- ltop.facetted(nrow(unique(model.frame(~color, ggplot2::diamonds)))) ggplot2::ggplot(ggplot2::diamonds, ggplot2::aes(x = carat, fill = cut)) + ggplot2::geom_density(position = "stack") + ggplot2::facet_wrap(~ color) + ggplot2::theme( legend.position = "inside", legend.justification = c(0,1), legend.background = ggplot2::element_blank(), legend.position.inside = ltop)

Furthermore, knowing that you have n free panels, you can play with e.g. ggplot2::guides(color = ggplot2::guide_legend(ncol = ...f(n)...) to specify the number of columns for use in you legend, etc.

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