How can I make the legend color bar the same height as my plot panel?

Question

I have generated a simple plot in R that shows the correlation coefficients for a set of data. Currently, the legend colorbar on the right side of the plot is a fraction of the entire plot size.

I would like the legend colorbar to be same height as the plot. I thought that I could use the legend.key.height to do this, but I have found that is not the case. I investigated the grid package unit function and found that there were some normalized units in there but when I tried them (unit(1, "npc")), the colorbar was way too tall and went off the page.

How can I make the legend the same height as the plot itself?

A full self contained example is below:

library(ggplot2)

corrs <- structure(list(Var1 = structure(c(1L, 2L, 3L, 1L, 2L, 3L, 1L, 2L, 3L), levels = c("Var1", "Var2", "Var3"), class = "factor"), Var2 = structure(c(1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 3L), levels = c("Var1", "Var2", "Var3"), class = "factor"), value = c(1, -0.11814395012334, -0.91732952510938, -0.969618394505233, 1, -0.00122085912153125, -0.191116513684392, -0.0373711776919663, 1)), class = "data.frame", row.names = c(NA, -9L))

ggplot(corrs, aes(x = Var1, y = Var2, fill = value)) +
  geom_tile() +
  theme(
    panel.border = element_blank(),
    axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1),
    aspect.ratio = 1,
    legend.position = "right",
    legend.key.height = unit(1, "inch")
  )

^{Created on 2022-12-29 with reprex v2.0.2}

Answer 1

Edit Updating to ggplot v3.0.0

This is messy, but based on this answer, and delving deeper into the ggplot grob, the legend can be positioned precisely.

# Load the needed libraries
library(ggplot2)
library(gtable)  # 
library(grid)
library(scales)
library(reshape2)


# Generate a collection of sample data
variables = c("Var1", "Var2", "Var3")
data = matrix(runif(9, -1, 1), 3, 3)
diag(data) = 1
colnames(data) = variables
rownames(data) = variables

# Generate the plot
corrs = data
plot  = ggplot(melt(corrs), aes(x = Var1, y = Var2, fill = value)) + 
   geom_tile() +
   theme_bw() + 
   theme(panel.border = element_blank()) +
   theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1)) +
   theme(aspect.ratio = 1) +
   # theme(legend.position = "right", legend.key.height = unit(1, "inch")) +
   labs(x = "", y = "", fill = "", title = "Correlation Coefficients") +
   scale_fill_gradient2(limits = c(-1, 1), breaks = c(-1, -.5, 0, .5, 1), expand = c(0,0), 
      low = muted("red"), mid = "black", high = muted("blue")) +  # Modified line
   geom_text(parse = TRUE, aes(label = sprintf("%.2f", value)), size = 3, color = "white") +
   scale_x_discrete(expand = c(0,0)) +  # New line
   scale_y_discrete(expand = c(0,0))    # New line
plot

# Get the ggplot grob
gt = ggplotGrob(plot)

# Get the legend
leg = gtable_filter(gt, "guide-box")

# Raster height
leg[[1]][[1]][[1]][[1]][[1]][[2]]$height = unit(1, "npc")

# Positions for labels and tick marks - five breaks, therefore, five positions
pos = unit.c(unit(0.01,"npc"), unit(.25, "npc"), unit(.5, "npc"), unit(.75, "npc"), unit(.99, "npc"))

# Positions the labels 
leg[[1]][[1]][[1]][[1]][[1]][[3]]$children[[1]]$y = pos

# Positions the tick marks
leg[[1]][[1]][[1]][[1]][[1]][[5]]$y0 = pos
leg[[1]][[1]][[1]][[1]][[1]][[5]]$y1 = pos

# Legend key height ?
leg[[1]][[1]][[1]][[1]]$heights = unit.c(rep(unit(0, "mm"), 3),
                                         unit(1, "npc"),
                                         unit(0, "mm"))
# Legend height
leg[[1]][[1]]$heights[[3]] = sum(rep(unit(0, "mm"), 3),
                                 unit(1, "npc"),
                                 unit(0, "mm"))

# grid.draw(leg)  # Check on heights and y values

# gtable_show_layout(gt) # Manually locate position of legend in layout
gt.new = gtable_add_grob(gt, leg, t = 7, l = 9)

# Draw it
grid.newpage()
grid.draw(gt.new)

Answer 2

It seems quite tricky, the closest I got was this,

## panel height is 1null, so we work it out by subtracting the other heights from 1npc
## and 1line for the default plot margins

panel_height <- unit(1,"npc") - sum(ggplotGrob(plot)[["heights"]][-3]) - unit(1,"line")
plot + guides(fill= guide_colorbar(barheight=panel_height))

unfortunately the vertical justification is a bit off.

Answer 3

The following option is a function that can be added to a ggplot to take the plot and make the colorbar the same height as the panel.

Essentially it uses the same technique as Baptiste, but is a bit more robust to changes in the ggplot implementation, and moves the legend title to a more natural position, allowing neater alignment. It also allows more recognisable ggplot-style syntax.

make_fullsize <- function() structure("", class = "fullsizebar")

ggplot_add.fullsizebar <- function(obj, g, name = "fullsizebar") {
  h <- ggplotGrob(g)$heights
  panel <- which(grid::unitType(h) == "null")
  panel_height <- unit(1, "npc") - sum(h[-panel])
  
  g + 
    guides(fill = guide_colorbar(barheight = panel_height,
                                 title.position = "right")) +
    theme(legend.title = element_text(angle = -90, hjust = 0.5))
}

You can then do:

ggplot(corrs, aes(x = Var1, y = Var2, fill = value)) + 
  geom_tile() +
  coord_cartesian(expand = FALSE) +
  make_fullsize()

The drawback is that the plot needs re-drawn if the plotting window is resized after the plot is drawn. This is a bit of a pain, but it's a fairly quick-and-simple fix. It will still work well with ggsave.

Note that the color bar is the same height as the panel, which is why it looks a bit neater to turn the expansion off in coord_cartesian, so it matches the actual tiles of the heatmap.

Another example for one of the linked duplicates:

library(reshape2)

dat <- iris[,1:4]
cor <- melt(cor(dat, use="p"))

ggplot(data=cor, aes(x=Var1, y=Var2, fill=value)) +
  geom_tile() + 
  labs(x = "", y = "") + 
  scale_fill_gradient2(limits=c(-1, 1)) +
  coord_cartesian(expand = FALSE) +
  make_fullsize()

Answer 4

The problem is that the plot panel does not have defined dimensions until drawing ("NULL unit"), but your legend guide has. See also npc coordinates of geom_point in ggplot2 or figuring out panel size given dimensions for the final plot in ggsave (to show count for geom_dotplot). I think it will be extremely tricky to draw the legend guide in the exact same size as your panel.

However, you can make use of a trick when dealing with complex legend formatting: Create a fake legend. The challenge here is to adjust the fill scale to perfectly match the range of your plot (which is not usually exactly the range of your data values). The rest is just a bit of R semantics. Some important comments in the code.

library(ggplot2)

corrs <- structure(list(Var1 = structure(c(1L, 2L, 3L, 1L, 2L, 3L, 1L, 2L, 3L), levels = c("Var1", "Var2", "Var3"), class = "factor"), Var2 = structure(c(1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 3L), levels = c("Var1", "Var2", "Var3"), class = "factor"), value = c(1, -0.11814395012334, -0.91732952510938, -0.969618394505233, 1, -0.00122085912153125, -0.191116513684392, -0.0373711776919663, 1)), class = "data.frame", row.names = c(NA, -9L))
## to set the scale properly, you will need to set limits and breaks, 
## I am doing this semi-automatically
range_fill <- range(corrs$value)
lim_fill <- c(floor(range_fill[1]), ceiling(range_fill[2]))
## len = 5 and round to 2 is hard coded, depending on the scale breaks that you want
breaks_fill <- round(seq(lim_fill[1], lim_fill[2], len = 5), 2)
## need to rescale the fill to the range of you y values,
## so that your fill scale correctly corresponds to the range of your y 
## however, the actual range of your plot depends if you're in a discrete or continuous range.
## here in a discrete range
lim_y <- range(as.integer(corrs$Var2))
lim_x <- range(as.integer(corrs$Var1))
lim_vals <- lim_y + c(-.5, .5)
## actual rescaling happens here
new_y <- scales::rescale(breaks_fill, lim_vals)
## the position and width of the color bar are defined with
scl_x <- lim_x[2] + .7 # the constant is hard coded
scl_xend <- scl_x + .2 # also hard coded
##  make a data frame for the segments to be created
## using approx so that we have many little segments
approx_fill <- approx(new_y, breaks_fill, n = 1000)
df_seg <- data.frame(y = approx_fill$x, color = approx_fill$y)
## data frame for labels, xend position being hard coded
df_lab <- data.frame(y = new_y, x = scl_xend + .1, label = breaks_fill)
## data frame for separators
sep_len <- .05
df_sep <- data.frame(
  y = new_y, yend = new_y,
  x = rep(c(scl_x, scl_xend - sep_len), each = 5),
  xend = rep(c(scl_x + sep_len, scl_xend), each = 5)
)

ggplot(corrs) +
  geom_tile(aes(x = Var1, y = Var2, fill = value)) +
  geom_segment(
    data = df_seg,
    aes(x = scl_x, xend = scl_xend, y = y, yend = y, color = color)
  ) +
  ## now the labels, the size being hard coded
  geom_text(data = df_lab, aes(x, y, label = label), size = 9 * 5 / 14) +
  ## now make the white little separators
  geom_segment(
    data = df_sep, aes(x = x, xend = xend, y = y, yend = yend),
    color = "white"
  ) +
  ## set both color and fill scales exactly
  scale_fill_continuous(limits = lim_fill, breaks = breaks_fill) +
  scale_color_continuous(limits = lim_fill, breaks = breaks_fill) +
  ## turn off coordinate clipping and limit panel to data area)
  coord_cartesian(xlim = lim_x, ylim = lim_y, clip = "off") +
  ## last but not least remove the other legends and add a bit of margin
  theme(
    legend.position = "none",
    plot.margin = margin(r = 1, unit = "in")
  )

^{Created on 2022-12-29 with reprex v2.0.2}