R.中的geom_text + ggmap为什么不起作用？

Question

我想添加一些标签来映射，我不知道为什么这不起作用，因为它应该：

library(ggplot2)
library(ggmap)

dt <- data.frame(
  n=c('0.07%','0.01%','0.02%','0.01%','0.03%','0.06%','0.04%','0.05%','0.06%','0.04%')
  ,lon=c(21.04,19.07,17.17,19.89,18.06,20.20,19.69,20.24,22.94,19.50)
  ,lat=c(52.23,53.90,50.49,50.18,54.33,51.73,52.00,52.19,51.20,51.78)
  )

map <- qmap('Poland',zoom=6)

map + geom_text(dt,aes(x=as.numeric(lon),y=as.numeric(lat),label=n))