If you aren't set on using ggplot2 for everything, you can sidestep fortify()
and just use the sp package's well-developed tools for plotting objects of class SpatialPolygons*
:
library(rgdal) # For readOGR(), generally preferable to readShapePoly() IMHO
library(spdep) # For the "columbus.shp" file that ships with the package
map <- readOGR(dsn = system.file("etc/shapes", package="spdep"),
layer = "columbus")
plot(map, border="red", lwd=3)