r - 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

EDIT: updating for ggplot version 0.9.2

The original answer (see below) broke at about version 0.9.0 or 0.9.1. The following works in 0.9.2

# 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)

# A plot
library(ggplot2)
p = ggplot(data = df, aes( x = x, y = y, label = Count, size = Count)) + 
   geom_point(colour = NA) +
   geom_text(show.legend = FALSE) +  
   guides(size = guide_legend(override.aes = list(colour = "black", shape = utf8ToInt("N")))) +
   scale_size(range = c(2, 10))

p

Original answer Answering my own question and using the snippet of code in @kohske's comment above:

# 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)

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

library(grid)
grid.gedit("^key-[-0-9]+$", label = "N")