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

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

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))

https://i.stack.imgur.com/Ape84.png

it's not too hard to add grobs to the gtable and make a fancy title that way,

library(ggplot2)
library(grid)
library(gridExtra)
library(magrittr)
library(gtable)

p <- ggplot() + 
  theme(plot.margin = unit(c(0.5, 1, 1, 1), "cm"))

lg <- list(textGrob("Rainfall", x=0, hjust=0, 
                    gp = gpar(fontsize=24, fontfamily="Skia", face=2, col="turquoise4")),
               textGrob("location", x=0, hjust=0, 
                        gp = gpar(fontsize=14, fontfamily="Zapfino", fontface=3, col="violetred1")),
           pointsGrob(pch=21, gp=gpar(col=NA, cex=0.5,fill="steelblue")))

margin <- unit(0.2, "line")
tg <- arrangeGrob(grobs=lg, layout_matrix=matrix(c(1,2,3,3), ncol=2),
                  widths = unit.c(grobWidth(lg[[1]]), unit(1,"null")),
                  heights = do.call(unit.c, lapply(lg[c(1,2)], grobHeight)) + margin)

grid.newpage()
ggplotGrob(p) %>%
  gtable_add_rows(sum(tg$heights), 0) %>%
  gtable_add_grob(grobs=tg, t = 1, l = 4)  %>%
  grid.draw()

https://i.stack.imgur.com/ozDSD.png

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"), "")))).

This version uses a gtable function. It allows two lines of text in the title. The text, size, colour, and font face of each line can be set independently of the other. However, the function will modify a plot with a single plot panel only.

Minor edit: Updating to ggplot2 v2.0.0

# The original plot
library(ggplot2)

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", stat = "identity") + 
     labs(x = "Weather    stations", y = "Accumulated Rainfall [mm]") + 
     theme(axis.text.x = element_text(angle = -45, hjust = 0, vjust = 1))


# The function to set text, size, colour, and face
plot.title = function(plot = NULL, text.1 = NULL, text.2 = NULL, 
   size.1 = 12,  size.2 = 12,
   col.1 = "black", col.2 = "black", 
   face.1 = "plain",  face.2 = "plain") {

library(gtable)
library(grid)

gt = ggplotGrob(plot)

text.grob1 = textGrob(text.1, y = unit(.45, "npc"), 
   gp = gpar(fontsize = size.1, col = col.1, fontface = face.1))
text.grob2 = textGrob(text.2,  y = unit(.65, "npc"), 
   gp = gpar(fontsize = size.2, col = col.2, fontface = face.2))

text = matrix(list(text.grob1, text.grob2), nrow = 2)
text = gtable_matrix(name = "title", grobs = text, 
   widths = unit(1, "null"), 
   heights = unit.c(unit(1.1, "grobheight", text.grob1) + unit(0.5, "lines"), unit(1.1,  "grobheight", text.grob2) + unit(0.5, "lines")))

gt = gtable_add_grob(gt, text, t = 2, l = 4)
gt$heights[2] = sum(text$heights)

class(gt) =  c("Title", class(gt))

gt
}

# A print method for the plot
print.Title <- function(x) {
   grid.newpage()   
   grid.draw(x)
}


# Try it out - modify the original plot
p = plot.title(m, "Rainfall", "Location", 
   size.1 = 20, size.2 = 15, 
   col.1 = "red", col.2 = "blue", 
   face.2 = "italic")

p

https://i.stack.imgur.com/E7Jk1.png

You could use wrap the plot in grid.arrange and pass a custom grid-based title,

https://i.stack.imgur.com/ixXoL.png

library(ggplot2)
library(gridExtra)

p <- ggplot() + 
  theme(plot.margin = unit(c(0.5, 1, 1, 1), "cm"))

tg <- grobTree(textGrob("Rainfall", y=1, vjust=1, gp = gpar(fontsize=25, face=2, col="black")),
               textGrob("location", y=0, vjust=0, gp = gpar(fontsize=12, face=3, col="grey50")),
               cl="titlegrob")
heightDetails.titlegrob <- function(x) do.call(sum,lapply(x$children, grobHeight))

grid.arrange(p, top = tg)

You might have noticed that Sandy's code doesn't produce a bold title for "Rainfall" - the instruction to make this bold should occur within the atop() function rather than the theme() function.

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(bold("Rainfall"), atop(italic("Location"), "")))) +
 theme(axis.text.x = element_text(angle=-45, hjust=0, vjust=1),
 plot.title = element_text(size = 25, colour = "black", vjust = -1))

https://i.stack.imgur.com/yUflw.png