Choropleth map in ggplot with polygons that have holes

Question

I\'m trying to draw a choropleth map of Germany showing poverty rate by state (inspired by this question).

The problem is that some of the states (Berlin, for examp

Answer 1

Just to add another small improvement to @Ista's and @jhoward's answers (thanks a lot for your help!).

The modification of @jhoward could be easily wrapped in a small function like this

gghole <- function(fort){
        poly <- fort[fort$id %in% fort[fort$hole,]$id,]
        hole <- fort[!fort$id %in% fort[fort$hole,]$id,]
        out <- list(poly,hole)
        names(out) <- c('poly','hole')
        return(out)
} 
# input has to be a fortified data.frame

Then, one doesn't need to recall every time how to extract holes info. The code would look like

    ggplot(map.df, aes(x=long, y=lat, group=group)) +
            geom_polygon(data=gghole(map.df)[[1]],aes(fill=poverty),colour="grey50")+
            geom_polygon(data=gghole(map.df)[[2]],aes(fill=poverty),colour="grey50")+
    # (optionally). Call by name
    #         geom_polygon(data=gghole(map.df)$poly,aes(fill=poverty),colour="grey50")+
    #         geom_polygon(data=gghole(map.df)$hole,aes(fill=poverty),colour="grey50")+
            scale_fill_gradientn(colours=brewer.pal(5,"OrRd"))+
            labs(x="",y="")+ theme_bw()+
            coord_fixed()