Changing the symbol in the legend key in ggplot2

Question

How do I change the geom_text legend key symbol? In the example below, I'd like to change the symbol in the legend key from a lower case "a" to, say, an upper case "N". I've looked at an example for doing something similar here, but couldn't get that example to work.

# Some toy data
df <- expand.grid(x = factor(seq(1:5)), y = factor(seq(1:5)), KEEP.OUT.ATTRS = FALSE)
df$Count = seq(1:25)

# An example plot
library(ggplot2)
ggplot(data = df, aes( x = x, y = y, label = Count, size = Count)) + 
   geom_text() +
   scale_size(range = c(2, 10))

Answer 1

With gtable version 0.2.0 (ggplot2 v 2.1.0) installed, Kohske's original solution (see the comments) can be made to work.

# Some toy data
df <- expand.grid(x = factor(seq(1:5)), y = factor(seq(1:5)), KEEP.OUT.ATTRS = FALSE)
df$Count = seq(1:25)

# Load packages
library(ggplot2)
library(grid)

# A plot
p = ggplot(data = df, aes( x = x, y = y, label = Count, size = Count)) + 
    geom_text() +
    scale_size(range = c(2, 10))
p

grid.ls(grid.force()) 
grid.gedit("key-[-0-9]-1-1", label = "N")

Or, to work on a grob object:

# Get the ggplot grob
gp = ggplotGrob(p)
grid.ls(grid.force(gp)) 

# Edit the grob
gp = editGrob(grid.force(gp), gPath("key-[1-9]-1-1"), grep = TRUE, global = TRUE,  
         label = "N")

# Draw it
grid.newpage()
grid.draw(gp)

Another option

Modify the geom

# Some toy data
df <- expand.grid(x = factor(seq(1:5)), y = factor(seq(1:5)), KEEP.OUT.ATTRS = FALSE)
df$Count = seq(1:25)

# Load packages
library(ggplot2)
library(grid)

# A plot
p = ggplot(data = df, aes( x = x, y = y, label = Count, size = Count)) + 
    geom_text() +
    scale_size(range = c(2, 10))
p

GeomText$draw_key <- function (data, params, size) { 
   pointsGrob(0.5, 0.5, pch = "N", 
   gp = gpar(col = alpha(data$colour, data$alpha), 
   fontsize = data$size * .pt)) }

p