使用geom_polygon绘制带孔的ggplot多边形

Question

There are questions out there about the fact that ggplot2 can't plot polygon shapes that have holes .

That is because, if the order of points is not OK, the end graph looks bad, usually with clipping/trimming lines inside the donut shape.

I have read a lot about how order matters, but I am not able to step forward. I have a SpatialPolygonsDataFrame with 26 features (comes from raster::rasterToPolygons(dissolve=T) ) and I want to plot it with ggplot .

Here's what happens -

r3.pol <- rasterToPolygons(r3, dissolve=T)
r3.df <- fortify(r3.pol)
names(r3.df) <- c('x','y','order','hole','piece','ID','group')
p <- ggplot(r3.df)
p <- p + geom_polygon(mapping=aes(x=x,y=y,group=ID), fill='red')
p <- p + coord_equal()

I see this output:

While it should be like so, with plot(r3.pol) :

How can I make this work? I tried for hours but I am not able to reorder r3.df . Also, can the information in r3.df$hole be helpful? It is returned by the function fortify for points that are holes (I think).

Side question: how can I give you my r3.pol SpatialPolygonsDataFrame, so that you can try yourself? I remember seeing long, reproducible "dumps" of objects here, but I don't know how to do it.

I save d the polygons data frame here . Was not able to save it using dput , sorry. You can fetch it using load .

Answer 1

I suggest to install the package "ggpolypath" and use geom_polypath instead of geom_polygon. Works for me.

Answer 2

My temporary solution is: @#$% polygons, and use the raster package.

Namely:

r <- raster(x=extent(r3.pol), crs=crs(r3.pol)) # empty raster from r3.pol
res(r) <- 250 # set a decent resolution (depends on your extent)
r <- setValues(r, 1) # fill r with ones
r <- mask(r, r3.pol) # clip r with the shape polygons

And now plot it as you would do with any raster with ggplot . The rasterVis package might come helpful here, but I'm not using it, so:

rdf <- data.frame(rasterToPoints(r))
p <- ggplot(rdf) + geom_raster(mapping=aes(x=x, y=y), fill='red')
p <- p + coord_equal()

And here it goes.

Alternatively, you can create the raster with rasterize , so the raster will hold the polygons values (in my case, just an integer):

r <- raster(x=extent(r3.pol), crs=crs(r3.pol))
res(r) <- 250
r <- rasterize(r3.pol, r)

rdf <- data.frame(rasterToPoints(r))
p <- ggplot(rdf) + geom_raster(mapping=aes(x=x, y=y, fill=factor(layer)))
p <- p + coord_equal()

If someone comes up with a decent solution for geom_polygon , probably involving re-ordering of the polygons data frame, I'll be glad to consider it.