ggplot2: Adjust the symbol size in legends

Question

How should I change the size of symbols in legends? I checked the document of theme but found no answer.

Here is an example:

library(gg         


        

Answer 1

You can make these kinds of changes manually using the override.aes argument to guide_legend():

g <- g + guides(shape = guide_legend(override.aes = list(size = 5)))
print(g)

Answer 2

You should use:

theme(legend.key.size = unit(3,"line"))

Answer 3

If you want to change the sizes of 2 components of a legend independently, it gets trickier, but it can be done by manually editing the individual components of the plot using the grid package.

Example based on this SO answer:

set.seed(1)
dat <- data.frame(x = runif(n = 100),
                  x2 = factor(rep(c('first', 'second'), each = 50)))
set.seed(1)
dat$y = 5 + 1.8 * as.numeric(dat$x2) + .3 * dat$x + rnorm(100)

# basic plot
g <- ggplot(data = dat,
       aes(x = x, y = y, color = x2))+
   theme_bw()+
   geom_point()+
   geom_smooth(method = 'lm')

# make the size of the points & lines in the legend larger
g + guides(color = guide_legend(override.aes = list(size = 2)))

# Make JUST the legend points larger without changing the size of the legend lines:
# To get a list of the names of all the grobs in the ggplot
g
grid::grid.ls(grid::grid.force())

# Set the size of the point in the legend to 2 mm
grid::grid.gedit("key-[-0-9]-1-1", size = unit(4, "mm"))

# save the modified plot to an object
g2 <- grid::grid.grab()
ggsave(g2, filename = 'g2.png')

Answer 4

Marius's answer did not work for me as of R version 3.2.2. You can still call guide_legend() with the same override.eas argument but you will need to specify color instead of shape in the wrapper function.

So if you're running a later version of R, try this instead:

g + guides(color = guide_legend(override.aes = list(size=5)))

EDIT

As pointed out by @Ibo in the comment, this may have been due to the color scale in the ggplot object. If the ggplot object has contains a color scale, the mapping of size (size=5) has to be set on the color instead.