世界地圖：
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)
中國地圖：
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")#
合並數據到地圖數據：
states <- map_data("state")##將地圖數據轉為數據框
arrests <- USArrests ##新數據（要與地圖數據中的某一列匹配）
names(arrests) <- tolower(names(arrests)) ##將列名翻譯為小寫，因為state數據的region為小寫
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") ##多邊形
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))) ##計算區的中心位置
ggplot(ia, aes(long, lat)) +
geom_polygon(aes(group = group),
fill = NA, colour = "grey60") +
geom_text(aes(label = subregion), data = centres, ##對地圖標注
size = 2, angle = 45)
出現問題時：
dev.off()
