Add fortify() method for sfc and sfg

[yutannihilation]

It happens often that an sf object is converted to sfc as the result of some operation (e.g. UNION).

library(ggplot2)
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.2.3, proj.4 4.9.3

nc <- st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)

nc_outer_lines <- nc %>%
  st_union() %>%
  st_boundary()

nc_inner_lines <- nc %>%
  st_boundary() %>%
  st_union() %>%
  st_line_merge()

ggplot() +
  geom_sf(data = nc, colour = "transparent", fill = "lightgreen") +
  geom_sf(data = nc_inner_lines, colour = "black",  linetype = "11", size = 1) +
  geom_sf(data = nc_outer_lines, colour = "black", size = 1.5) +
  theme_minimal()
#> Error: `data` must be a data frame, or other object coercible by `fortify()`, not an S3 object with class sfc_MULTILINESTRING/sfc

Created on 2018-06-06 by the reprex package (v0.2.0).

It's not a big deal to convert them back to st_sf() manually like below, but it's nice and handy if they are converted automatically. As plot() can plot sfc and sfg objects, I expect ggplot2 can plot these as well.

ggplot() +
  geom_sf(data = nc, colour = "transparent", fill = "lightgreen") +
  geom_sf(data = st_sf(nc_inner_lines), colour = "black",  linetype = "11", size = 1) +
  geom_sf(data = st_sf(nc_outer_lines), colour = "black", size = 1.5) +
  theme_minimal()

(The expected result:)

[hadley]

Thanks!!

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