通过使用 plyr pacakage中的 ddply ，我们可以创建一个基于累积为每个标签获得正确的位置：

By using ddply from the plyr pacakage, we can create a new variable based on the cumulative sums to get the correct position for each label:

library(plyr)
df <- data.frame(
    uni = rep(c("D","E","F","G","H"),3),
    Var1 = factor(c(rep("A",5),rep("B",5),rep("C",5))),
    Freq = c(53.6,50.0,48.5,50.0,56.2,23.2,18.5,27.7,20.0,14.3,23.2,31.5,23.8,30.0,29.6))

df = ddply(df, .(uni), transform, labPosition = cumsum(Freq)-Freq/2)

df$label = paste(round(df$Freq,0),"%", sep = "")
ggplot(data = df, aes(x = uni, y = Freq, fill = Var1)) +
    geom_bar(stat = "identity", width = 1) +
    scale_fill_brewer(palette = 3) + 
    geom_text(aes(y = labPosition, label = label, position ="identity"), hjust=0.5, vjust=0.5, size = 1, face = "bold") + 
    xlab('') + 
    ylab('') + 
    labs(fill = '') + 
    ggtitle('Example') + 
    theme(axis.text.y = element_text(size=14,face="bold"), panel.background = element_blank(), plot.title = element_text(size = 20, colour = "black", face = "bold")) + 
    guides(size=FALSE)

这会按组创建累计和的新变量，然后减去频率本身由2中心在该段的中间。

This creates a new variable of the cumulative sum by group, and then subtracts the frequency itself divided by 2 to center it in the middle of that segment.