Different font faces and sizes within label text entries in ggplot2

Question

I am building charts that have two lines in the axis text. The first line contains the group name, the second line contains that group population. I build my axis labels as

Answer 1

I also think that I could not to make the graph by only using ggplot2 features.

I would use grid.text and grid.gedit.

require(ggplot2)
Treatment <- rep(c('T','C'), each=2)
Gender <- rep(c('Male','Female'), 2)
Response <- sample(1:100, 4)
test_df <- data.frame(Treatment, Gender, Response)

xbreaks <- levels(test_df$Gender)
xlabels <- paste(xbreaks,'\n',c('',''))

hist <- ggplot(test_df, aes(x=Gender, y=Response, fill=Treatment,
  stat="identity"))
hist + geom_bar(position = "dodge") + 
  scale_y_continuous(limits = c(0, 100), name = "") +
  scale_x_discrete(labels=xlabels, breaks = xbreaks) + 
  opts(axis.text.x = theme_text(face='bold', size=12))
grid.text(label="POP1", x = 0.29, y = 0.06)
grid.text(label="POP2", x = 0.645, y = 0.06)
grid.gedit("GRID.text", gp=gpar(fontsize=8))

Please try to tune a code upon according to your environment (e.g. the position of sub-axis labels and the fontsize).