polygon clipping in ggplot

ggfail.R

usq = data.frame(x=c(0,1,1,0),y=c(0,0,1,1))
    
pts1 = 
structure(list(x = c(-0.380226816911908, -0.368506824640174, 
0.276092750305198, 0.281952746441065, -0.350926836232573, -0.362646828504307, 
-0.720106592792195), y = c(-0.287042131866309, 0.237226050260459, 
0.276546163919967, 0.532126902706766, 0.578000368642859, 1.06294843711012, 
1.06294843711012)), .Names = c("x", "y"), row.names = c(NA, -7L
), class = "data.frame")

polys = SpatialPolygons(list(Polygons(list(Polygon(pts1)),ID=1),Polygons(list(Polygon(usq)),ID=2)))

plot(polys)

fp = fortify(polys)

# this works
ggplot(fp,aes(x=long,y=lat,colour=id, group=id)) + geom_polygon(fill=NA) 

# this mashes the clipped polygon:
ggplot(fp,aes(x=long,y=lat,colour=id, group=id)) + geom_polygon(fill=NA) +xlim(c(-0.4,1.2))+ylim(c(-0.4,1.2))

# this works:
ggplot(fp,aes(x=long,y=lat,colour=id,group=id,fill=id)) + geom_polypath()

# this fails with a "non-finite" error:
ggplot(fp,aes(x=long,y=lat,colour=id,group=id,fill=id)) + geom_polypath() +xlim(c(-0.4,1.2))+ylim(c(-0.4,1.2))

# using real data
require(spdep)
example(columbus)
plot(columbus, xlim=c(8.9,9.6), ylim=c(12.0,12.8))
# mashed up
ggplot(columbus,aes(x=long,y=lat,group=group)) + geom_polygon(fill=NA, colour="black") + xlim(c(8.9,9.6)) + ylim(c(12.0,12.8))

xlim()/ylim() sets the limits of the scale, not the limits of the coordinate system (you need to use coord_cartesian())

Please fix or remove so that future visitors don't wind up thinking that ggplot2 is the failure here.