问在ggplot2中，如何添加额外的图例？

提问于 2018-04-26 00:25:03

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我正在尝试ggplot2中使用独立数据框中的数据构建地图。代码如下：

library(maptools)

xx <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1], IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))

xx.sub1 <- subset(xx, xx$FIPSNO < 37010)
xx.sub2 <- subset(xx, xx$FIPSNO > 37010)

xx.sub1@data$id <- rownames(xx.sub1@data)
xx.sub1.points <- fortify(xx.sub1, region="id")
xx.sub1.df = join(xx.sub1.points, xx.sub1@data, by="id")

xx.sub2@data$id <- rownames(xx.sub2@data)
xx.sub2.points <- fortify(xx.sub2, region="id")
xx.sub2.df = join(xx.sub2.points, xx.sub2@data, by="id")

ggplot(xx.sub2.df) + 
  aes(long, lat, fill = (SID79/BIR79)*1000, group = group) + 
  geom_polygon() + geom_path(color="grey80") +
  coord_equal() + 
  scale_fill_gradientn(colours = brewer.pal(7, "YlOrBr")) +
  geom_polygon(data = xx.sub1.df, fill = "grey50") + 
  geom_path(data = xx.sub1.df, color="grey80") +
  labs(fill = "Mapped value", title = "Title")

到目前为止，一切都按预期工作，我得到一张不错的map：