轴标签不会使用qmplot打印在我的地图中

Question

我正在使用qmplot制作地图，但不会打印轴标签。知道为什么吗？

    library(ggmap)
    set.seed(123); d = data.frame(long=rnorm(20,-3,0.05), lat=rnorm(20,59,0.05))
    qmplot(long, lat, data = d, main = "This plot is based on random data",
    xlab = "Longitude", ylab = "Latitude")