how to snip or crop or white-fill a large. expanded (by 10%) rectangle outside of a polygon with ggplot2

Question

this question is a follow-up of my prior SO question and is related to this question.

i'm just trying to white-fill an area 10% bigger than a simple polygon with ggplot2. maybe i'm grouping things wrong? here's a photo of the spike with reproducible code below

# reproducible example
library(rgeos)
library(maptools)
library(raster)

shpct.tf <- tempfile() ; td <- tempdir()

download.file( 
    "ftp://ftp2.census.gov/geo/pvs/tiger2010st/09_Connecticut/09/tl_2010_09_state10.zip" ,
    shpct.tf ,
    mode = 'wb'
)

shpct.uz <- unzip( shpct.tf , exdir = td )

# read in connecticut
ct.shp <- readShapePoly( shpct.uz[ grep( 'shp$' , shpct.uz ) ] )

# box outside of connecticut
ct.shp.env <- gEnvelope( ct.shp )
ct.shp.out <- as( 1.2 * extent( ct.shp ), "SpatialPolygons" )


# difference between connecticut and its box
ct.shp.env.diff <- gDifference( ct.shp.env , ct.shp )
ct.shp.out.diff <- gDifference( ct.shp.out , ct.shp )


library(ggplot2)


# prepare both shapes for ggplot2
f.ct.shp <- fortify( ct.shp )
env <- fortify( ct.shp.env.diff )
outside <- fortify( ct.shp.out.diff )


# create all layers + projections
plot <- ggplot(data = f.ct.shp, aes(x = long, y = lat))  #start with the base-plot 

layer1 <- geom_polygon(data=f.ct.shp, aes(x=long,y=lat), fill='black')

layer2 <- geom_polygon(data=env, aes(x=long,y=lat,group=group), fill='white')

layer3 <- geom_polygon(data=outside, aes(x=long,y=lat,group=id), fill='white')

co <- coord_map( project = "albers" , lat0 = 40.9836 , lat1 = 42.05014 )

# this works
plot + layer1 

# this works
plot + layer2

# this works
plot + layer1 + layer2

# this works
plot + layer2 + co

# this works
plot + layer1 + layer3

# here's the problem: this breaks
plot + layer3 + co

# this also breaks, but it's ultimately how i want to display things
plot + layer1 + layer3 + co

# this looks okay in this example but
# does not work for what i'm trying to do-
# cover up points outside of the state
plot + layer3 + layer1 + co

Answer 1

This is because `coord_map', or more generally non-linear coordinates, internally interpolates vertices so that line is draw as a curve corresponding the coordinate. In your case, interpolation will be performed between a point of the outer rectangle and a point of inner edge, which you see as the break.

You can change this by:

co2 <- co
class(co2) <- c("hoge", class(co2))
is.linear.hoge <- function(coord) TRUE
plot + layer1 + layer3 + co2

You can also find the difference of behavior here:

ggplot(data.frame(x = c(0, 90), y = 45), aes(x, y)) + geom_line() + co + ylim(0, 90)

ggplot(data.frame(x = c(0, 90), y = 45), aes(x, y)) + geom_line() + co2 + ylim(0, 90)