如何防止ggmap（与Stamen贴图）不对齐点并在低变焦时显示纬度？

Question

当使用ggmap，Stamen贴图和大陆级缩放（zoom = 3）时，点纬度不与地图对齐。例如：

library(ggmap)
gc <- geocode('the white house')

qmap('the white house', zoom = 3) + # looks good
  geom_point(aes(x = lon, y = lat), data = gc, colour = 'red')

qmap('the white house', zoom = 3, source = 'stamen', maptype="toner") + # not so good
  geom_point(aes(x = lon, y = lat), data = gc, colour = 'red')

googlemaps和osm地图似乎已经出现了addressed缺陷 - 我可以采用哪些策略来解决Stamen的问题？