Draw a very simple prefectural choropleth map of Japan with ggplot

ggJapanPrefecturesMap.R

ggJapanPrefecturesMap <- function (col = "white", fill = NULL, inset = TRUE, silent = FALSE, ...) 
{
  require(Nippon)
  if(require(ggplot2)){
    require(foreign)
    m <- readShapePoly(system.file("shapes/jpn.shp", package = "Nippon")[1], 
                       proj4string = CRS("+proj=longlat +datum=WGS84"))
    if (inset) {
      xy.okinawa <- m@polygons[[47]]@Polygons[[1]]@coords
      xy.okinawa[, 1] <- xy.okinawa[, 1] + 7
      xy.okinawa[, 2] <- xy.okinawa[, 2] + 14
      m@polygons[[47]]@Polygons[[1]]@coords <- xy.okinawa
      labpt <- m@polygons[[47]]@labpt
      labpt[1] <- labpt[1] + 7
      labpt[2] <- labpt[2] + 14
      m@polygons[[47]]@labpt <- labpt
      m@bbox[1, 1] <- 130
      m@bbox[2, 1] <- 31
    }
    fm <- fortify(m)
    fm$id <- as.numeric(fm$id)
    d <- read.dbf(system.file("shapes/jpn.dbf", package="Nippon"), as.is=TRUE)
    d$id <- as.numeric(d$SP_ID) - 1
    g <- ggplot(d) + theme_bw()
    if(is.null(fill)){
      col <- head(rep(col, ceiling(47 / length(col))), 47)
      g <- g + geom_map(map = fm, aes(map_id = id), fill = col, colour = "black", size = 0.2)
    }else{
      g <- g + geom_map(map = fm, aes(map_id = id, fill = fill), colour = "black", size = 0.2)
    }
    g <- g + expand_limits(x = fm$long, y = fm$lat) + coord_map() +
      theme(axis.title = element_blank())
    if (inset) {
      g <- g + geom_line(data = data.frame(long = c(132, 135, 137, 137), lat = c(38, 38, 40, 43)),
                         mapping = aes(x = long, y = lat), colour = "black", size = 0.2)
    }
    if(!silent)plot(g)
    coord <- as.data.frame(coordinates(m))
    colnames(coord) <- c("long", "lat")
    return(invisible(list(plot = g, coordinates = coord)))
  }else{
    return(JapanPrefecturesMap(col = col, inset = inset, ...))
  }
}