世界地图：

library(maps)
data("world.cities")
bigcities <- subset(world.cities, pop > 5000000)
qplot(long, lat, data = bigcities,colour=country.etc,size=pop)+ borders("world", size= 0.5)

中国地图：

library(mapdata)

ch_cities <- subset(world.cities, country.etc=="China")
ggplot(ch_cities, aes(long, lat)) + geom_point(colour= alpha("red",0.5))+ borders("china")#

合并数据到地图数据：

　　

states <- map_data("state")##将地图数据转为数据框
arrests <- USArrests ##新数据（要与地图数据中的某一列匹配）
names(arrests) <- tolower(names(arrests)) ##将列名翻译为小写，因为state数据的region为小写
arrests$region <- tolower(rownames(USArrests)) ##增添新列 region

choro <- merge(states, arrests, by = "region")

# Reorder the rows because order matters when drawing polygons
# and merge destroys the original ordering
choro <- choro[order(choro$order), ]
qplot(long, lat, data = choro, group = group, 
  fill = Assault, geom = "polygon") ##多边形
qplot(long, lat, data = choro, group = group, 
  fill = Assault / murder, geom = "polygon")


ia <- map_data("county", "iowa")
library(plyr)
mid_range <- function(x) mean(range(x, na.rm = TRUE))
centres <- ddply(ia, .(subregion), 
  colwise(mid_range, .(lat, long)))  ##计算区的中心位置
ggplot(ia, aes(long, lat)) + 
  geom_polygon(aes(group = group), 
    fill = NA, colour = "grey60") +
  geom_text(aes(label = subregion), data = centres,  ##对地图标注
    size = 2, angle = 45)


出现问题时：
dev.off()

　　

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