Multirow axis labels with nested grouping variables

Question

I would like the levels of two different nested grouping variables to appear on separate lines below the plot, and not in the legend. What I have right now is this code:

Answer 1

@agstudy already answered this question and I'm going to use it myself, but if you'd accept something uglier, but simpler, this is what I came with before his answer:

data <- read.table(text = "Group Category Value
    S1 A   73
    S2 A   57
    S1 B   7
    S2 B   23
    S1 C   51
    S2 C   87", header=TRUE)

p <- ggplot(data=data, aes(x=Category, y=Value, fill=Group))
p + geom_bar(position = 'dodge') +
  geom_text(aes(label=paste(Value, "%")), position=position_dodge(width=0.9),   vjust=-0.25) +
  geom_text(colour="darkgray", aes(y=-3, label=Group),  position=position_dodge(width=0.9), col=gray) +
  theme(legend.position = "none", 
    panel.background=element_blank(),
    axis.line = element_line(colour = "black"),
    axis.line.x = element_line(colour = "white"),
    axis.ticks.x = element_blank(),
    panel.grid.major = element_blank(),
    panel.grid.minor = element_blank(),
    panel.border = element_blank(),
    panel.background = element_blank()) +
  annotate("segment", x = 0, xend = Inf, y = 0, yend = 0)

Which will give us: