我正在使用stat_density2d()ggplot2 的功能在城市地图下绘制密度核图。问题是城市周围有海洋，我希望图形的颜色尊重多边形的限制，即不要逆着海洋前进。可能，为了实现这一点，有必要改变“影子”的大小。对不起数据的大小，但这只是我原始地图的一个较小的多边形。但基本上是经纬度和ggplot2代码之后。
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group<- rep(0.1,length(long))
p3<-data.frame(long=c(-38.41799,-38.41193,-38.41927,-38.42362,-38.41234,-38.41330,-38.40912),
lat =c(-12.96622,-12.96708,-12.96775,-12.95499,-12.95798,-12.95816,-12.95501))
ggplot() + geom_polygon(aes(x = long, y = lat, group=group), colour = "black",fill="white")+
geom_polygon() + theme_bw()+ theme(panel.background = element_rect(fill = "light blue"))+
stat_density2d(aes(x=long, y=lat, fill = ..level..), alpha=0.25,data=as.data.frame(p3), geom="polygon")+
scale_fill_gradientn(colours=rev(brewer.pal(11,"Spectral")), name="Kernel density",breaks=c(0,200),
labels=c("Min","Max"),limits=c(0,200))
输出是：
