From ggplot 2.2.0 labels can easily be stacked by using position = position_stack(vjust = 0.5) in geom_text.
ggplot(Data, aes(x = Year, y = Frequency, fill = Category, label = Frequency)) +
geom_bar(stat = "identity") +
geom_text(size = 3, position = position_stack(vjust = 0.5))
Also note that "position_stack() and position_fill() now stack values in the reverse order of the grouping, which makes the default stack order match the legend."
Answer valid for older versions of ggplot:
Here is one approach, which calculates the midpoints of the bars.
library(ggplot2)
library(plyr)
# calculate midpoints of bars (simplified using comment by @DWin)
Data <- ddply(Data, .(Year),
transform, pos = cumsum(Frequency) - (0.5 * Frequency)
)
# library(dplyr) ## If using dplyr...
# Data <- group_by(Data,Year) %>%
# mutate(pos = cumsum(Frequency) - (0.5 * Frequency))
# plot bars and add text
p <- ggplot(Data, aes(x = Year, y = Frequency)) +
geom_bar(aes(fill = Category), stat="identity") +
geom_text(aes(label = Frequency, y = pos), size = 3)
