How can we make xkcd style graphs?

You might want to consider the following package:

Package xkcd: Plotting ggplot2 graphics in a XKCD style.

library(xkcd)
vignette("xkcd-intro")

Some examples (Scatterplots, Bar Charts):

• Scatterplot:

• Bar Chart:

Basic line-drawing function:

xkcd_line <- function(x, y, color) {
  len <- length(x);
  rg <- par("usr");
  yjitter <- (rg[4] - rg[3]) / 1000;
  xjitter <- (rg[2] - rg[1]) / 1000;
  x_mod <- x + rnorm(len) * xjitter;
  y_mod <- y + rnorm(len) * yjitter;
  lines(x_mod, y_mod, col='white', lwd=10);
  lines(x_mod, y_mod, col=color, lwd=5);
}

Basic axis:

xkcd_axis <- function() {
  rg <- par("usr");
  yaxis <- 1:100 / 100 * (rg[4] - rg[3]) + rg[3];
  xaxis <- 1:100 / 100 * (rg[2] - rg[1]) + rg[1];
  xkcd_line(1:100 * 0 + rg[1] + (rg[2]-rg[1])/100, yaxis,'black')
  xkcd_line(xaxis, 1:100 * 0 + rg[3] + (rg[4]-rg[3])/100, 'black')
}

And sample code:

data <- data.frame(x=1:100)
data$one <- exp(-((data$x - 50)/10)^2)
data$two <- sin(data$x/10)
plot.new()
plot.window(
    c(min(data$x),max(data$x)),
    c(min(c(data$one,data$two)),max(c(data$one,data$two))))
xkcd_axis()
xkcd_line(data$x, data$one, 'red')
xkcd_line(data$x, data$two, 'blue')

Produces:

Thinking along the same line as some of the other answers, I've "un-ggplotted" the chart and also added on the flexibility of the x-axis label locations (which seems to be common in xkcd) and an arbitrary label on the chart.

Note that I had a few issues with loading the Humor Sans font and manually downloaded it to working directory.

And the code...

library(ggplot2)
library(extrafont)

### Already have read in fonts (see previous answer on how to do this)
loadfonts()

### Set up the trial dataset 
data <- NULL
data$x <- seq(1, 10, 0.1)
data$y1 <- sin(data$x)
data$y2 <- cos(data$x)
data$xaxis <- -1.5

data <- as.data.frame(data)

### XKCD theme
theme_xkcd <- theme(
    panel.background = element_rect(fill="white"), 
    axis.ticks = element_line(colour=NA),
    panel.grid = element_line(colour="white"),
    axis.text.y = element_text(colour=NA), 
    axis.text.x = element_text(colour="black"),
    text = element_text(size=16, family="Humor Sans")
    )

 ### Plot the chart
 p <- ggplot(data=data, aes(x=x, y=y1))+
      geom_line(aes(y=y2), position="jitter")+
      geom_line(colour="white", size=3, position="jitter")+
      geom_line(colour="red", size=1, position="jitter")+
      geom_text(family="Humor Sans", x=6, y=-1.2, label="A SIN AND COS CURVE")+
      geom_line(aes(y=xaxis), position = position_jitter(h = 0.005), colour="black")+
      scale_x_continuous(breaks=c(2, 5, 6, 9), 
      labels = c("YARD", "STEPS", "DOOR", "INSIDE"))+labs(x="", y="")+
      theme_xkcd

ggsave("xkcd_ggplot.jpg", plot=p, width=8, height=5)

Here's an attempt at the fonts, based on links from the xkcd forums and the extrafont package:

As noted above there is a forum discussion about fonts on the xkcd site: I grabbed the first one I could find, there may be other (better?) options (@jebyrnes posts another source for possible fonts in comments above -- the TTF file is here; someone reported a 404 error for that source, you might alternatively try here or here, substituting those URLs appropriately for xkcdFontURL below; you may have to work a bit harder to retrieve the Github-posted links)

   xkcdFontURL <- "http://simonsoftware.se/other/xkcd.ttf"
   download.file(xkcdFontURL,dest="xkcd.ttf",mode="wb")

(This is for quickie, one-off use: for regular use you should put it in some standard system font directory.)

   library(extrafont)

The most useful information about fonts was on the extrafont github site -- this is taken from there

font_import(".")   ## because we downloaded to working directory
loadfonts()

Example taken more or less verbatim from the github site:

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point() +
  ggtitle("Fuel Efficiency of 32 Cars") +
  xlab("Weight (x1000 lb)") + ylab("Miles per Gallon") +
  theme(text=element_text(size=16, family="xkcd"))

ggsave("xkcd_ggplot.pdf", plot=p,  width=4, height=4)
## needed for Windows:
##   Sys.setenv(R_GSCMD = "C:/Program Files/gs/gs9.05/bin/gswin32c.exe")
embed_fonts("xkcd_ggplot.pdf")