ggplot format italic annotation

Question

Can I use markup in a ggplot annotation?

Let's say, I have this graph:

p <- function(i) 8*i
a <- function(i) 1+4*i*(i-1)

library(ggplot2)
library(reshape2)

i <- 1:(8*365/7)
d <- data.frame(i=i,p=p(i),a=sapply(i,a))
d <- melt(d, id.vars='i')
p <- ggplot(d, aes(i, value, linetype=variable)) +
    geom_hline(yintercept=700^2) +
    geom_line() +
    scale_linetype_manual(values=c(2,1)) +
    #geom_point() +
    scale_x_continuous(breaks=(0:20)*365/7, labels=0:20) +
    #scale_y_continuous(breaks=c(0,700^2), labels=c(0,expression(L^2)))
    scale_y_sqrt() +
    #scale_y_log10() +
    annotate('text', 8*365/7, 1e3, label="P(i)=8i", hjust=1, size=3) +
    annotate('text', 8*365/7, 2.5e5, label="A(i)=1+4i(i-1)", hjust=1, size=3)
print(p + theme_classic())

I know I can use fontface=3 and put everything in italic. But I do not want the numbers in italic, only the variable i. Preferably, P and A would not be in italic as well.

Any ideas?

Answer 1

Right now this page is the top search result on google for ggplot annotate italic. For the benefit of those who simply want to italicize an entire annotation, I'm writing this post. Use annotate's fontface option. Example:

seq(0,3.14,0.01)
qplot(x, sin(x)) +   # works the same for qplot or ggplot
annotate(geom = 'text', 
         x = 1.5, 
         y = 0.5, 
         hjust = 0.5,
         label = 'Hello, World', 
         fontface = 'italic')

Answer 2

Use parse=TRUE and supply a string formatted according to ?plotmath.

p <- ggplot(d, aes(i, value, linetype=variable)) +
    geom_hline(yintercept=700^2) +
    geom_line() +
    scale_linetype_manual(values=c(2,1)) +
    scale_x_continuous(breaks=(0:20)*365/7, labels=0:20) +
    scale_y_sqrt() +
    annotate('text', 8*365/7, 1e3,
             label="P(italic(i))==8~italic(i)", parse=TRUE,
             hjust=1, size=3) +
    annotate('text', 8*365/7, 2.5e5,
             label="A(italic(i))==1+4~italic(i)(italic(i)-1)", parse=TRUE, 
             hjust=1, size=3)