r - How can I make the legend in ggplot2 the same height as my plot?

Question

I have generated a simple plot in R (version R version 3.0.1 (2013-05-16)) using ggplot2 (version 0.9.3.1) 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:

# Load the needed libraries
library(ggplot2)
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
ggplot(melt(corrs), aes(x = Var1, y = Var2, fill = value)) +
  geom_tile() +
  geom_text(parse = TRUE, aes(label = sprintf("%.2f", value)), size = 3, color = "white") +
  theme_bw() +
  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")) +
        labs(x = "", y = "", fill = "", title = "Correlation Coefficients") +
        scale_fill_gradient2(limits = c(-1, 1), expand = c(0, 0),
                             low = muted("red"),
                             mid = "black",
                             high = muted("blue"))

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)