Q:

How to add a ggplot2 subtitle with different size and colour?

I'm using ggplot2 to improve precipitation barplots.
Here's a reproducible example of what I want to achieve: 
library(ggplot2)
library(gridExtra)
secu <- seq(1, 16, by=2)
melt.d <- data.frame(y=secu, x=LETTERS[1:8])
m <- ggplot(melt.d, aes(x=x, y=y)) +
  geom_bar(fill="darkblue") + 
  labs(x="Weather    stations", y="Accumulated Rainfall [mm]") +
  opts(axis.text.x=theme_text(angle=-45, hjust=0, vjust=1),
       title=expression("Rainfall"), plot.margin = unit(c(1.5, 1, 1, 1), "cm"),
       plot.title = theme_text(size = 25, face = "bold", colour = "black", vjust = 5))
z <- arrangeGrob(m, sub = textGrob("Location", x = 0, hjust = -3.5, vjust = -33, gp = gpar(fontsize = 18, col = "gray40"))) #Or guessing x and y with just option
z

I don't know how to avoid using guessing numbers on hjust and vjust on ggplot2? Is there a better way to put a subtitle (not just using \n, but a subtitle with different text color and size)?
I need to be able to use with ggsave to have a pdf file.
Here are two related questions:
Add a footnote citation outside of plot area in R?
How can I add a subtitle and change the font size of ggplot plots in R?
Thanks for any help.

A:

The latest ggplot2 builds (i.e., 2.1.0.9000 or newer) have subtitles and below-plot captions as built-in functionality. That means you can do this:
library(ggplot2) # 2.1.0.9000+ 

secu <- seq(1, 16, by=2)
melt.d <- data.frame(y=secu, x=LETTERS[1:8])

m <-  ggplot(melt.d, aes(x=x, y=y))
m <- m + geom_bar(fill="darkblue", stat="identity")
m <- m + labs(x="Weather    stations", 
              y="Accumulated Rainfall [mm]",
              title="Rainfall",
              subtitle="Location")
m <- m + theme(axis.text.x=element_text(angle=-45, hjust=0, vjust=1)) 
m <- m + theme(plot.title=element_text(size=25, hjust=0.5, face="bold", colour="maroon", vjust=-1))
m <- m + theme(plot.subtitle=element_text(size=18, hjust=0.5, face="italic", color="black"))
m

A:

Ignore this answer ggplot2 version 2.2.0 has title and subtitle functionality. See @hrbrmstr's answer below. 

You could use nested atop functions inside an expression to get different sizes. 
EDIT Updated code for ggplot2 0.9.3
m <-  ggplot(melt.d, aes(x=x, y=y)) + 
     geom_bar(fill="darkblue", stat = "identity") + 
     labs(x="Weather    stations", y="Accumulated Rainfall [mm]") + 
     ggtitle(expression(atop("Rainfall", atop(italic("Location"), "")))) +
     theme(axis.text.x = element_text(angle=-45, hjust=0, vjust=1), 
     #plot.margin = unit(c(1.5, 1, 1, 1), "cm"), 
     plot.title = element_text(size = 25, face = "bold", colour = "black", vjust = -1))

A:

It appears opts is deprecated as of ggplot 2 0.9.1 and no longer functional. This worked for me with the latest versions as of today: + ggtitle(expression(atop("Top line", atop(italic("2nd line"), "")))).

