Use the font Humor Sans instead of font xkcd with theme_xkcd()

ggplot2_xkcd_Humor_Sans.R

# The xkcd font used by the package xkcd (which provides a theme for ggplot2)
# is missing many characters and some characters don't seem to display correctly.
# An alternate xkcd-style font is Humor Sans, available free from 
# \url{http://antiyawn.com/uploads/humorsans.html}
# The code below forces the use of Humor Sans instead of xkcd.
# The xkcd and ggplot2 packages are available from CRAN.

library(ggplot2)
library(xkcd)

# Set up some data for plotting
temp_data2 <- data.frame(
  gap = rep(c(16, 32, 64), each = 2),
  eff = runif(3*2))

# Create a ggplot2 scatterplot. The slashes (/) in the
# axis labels do not print correctly using font xkcd,
# but display when using Humor Sans
ggplot(temp_data2, aes(x = gap, y = eff)) +
  geom_point() +
  ylab("efficiency / %/100") +
  xlab("disk gap / 1/64 inch") +
  theme_xkcd() +
  theme(text = element_text(size = 16, family = "Humor Sans"))