我得到这样的数据：

df = data.frame(x = rep(sample(letters,4),2),y = round(runif(8,1,100),0),z = c(rep("group1",rep("group2",4)))

# I then add a 'percent' column like so:

df$perc[1:4] = df$y[1:4] / sum(df$y[1:4])
df$perc[5:8] = df$y[5:8] / sum(df$y[5:8])

# Which I then convert like so:
df$perc = paste(round(df$perc * 100,1),"%",sep="")

# The ggplot:
library(ggplot2)

ggplot(df) + 
geom_bar(aes(z,y,fill=x),position="dodge",stat="identity") + 
geom_text(aes(z,label=perc),position=position_dodge(width=1),size=4)

结果：

我无法弄清楚我做错了什么.