R：刪除geom_polygon ggmap中的多余行

Question

我有產生這張地圖的代碼：

library(ggmap)
library(mapdata)

counties <- map_data("county")
iowa_counties <- subset(counties, region=="iowa")
sq_map2 <- get_map(location = c(-92.55191,42.89219),  maptype = "satellite", source = "google", zoom = 9)
ggmap(sq_map2) + 
scale_y_continuous(limits=c(42.51118, 43.26184), expand=c(0,0)) + 
scale_x_continuous(limits=c(-93.0735, -92.03318), expand=c(0,0)) +
geom_polygon(data = iowa_counties, aes(x=long, y=lat, group=group), fill = NA, color = "white")

如您所見，左上角還有一個三角形。 我該如何擺脫呢？

Answer 1

一種非常簡單的方法是減小y限制的最大值以刪除該上限並防止三角形的構造。 只需將scale_y_continuous的值從43.26184為43.25184就可以了：

library(ggmap)
library(mapdata)

counties <- map_data("county")
iowa_counties <- subset(counties, region=="iowa")
sq_map2 <- get_map(location = c(-92.55191,42.89219),  maptype = "satellite", source = "google", zoom = 9)
ggmap(sq_map2) + 
scale_y_continuous(limits=c(42.51118, 43.26184), expand=c(0,0)) + 
scale_x_continuous(limits=c(-93.0735, -92.03318), expand=c(0,0)) +
geom_polygon(data = iowa_counties, aes(x=long, y=lat, group=group), fill = NA, color = "white")

當然，您也可以過濾掉iowa_counties某個緯度以上的點。