symbols for different polygon layers in ggplot2

Question

I'm trying to plot the boundary of different spatialPolygonsDataFrames in ggplot2 or in rasterVis gplot(), but I just can't find the way, they tend to overwrite the previous one; this is something quite common in mapping, having different symbols for each layer, it's something like this what I'm trying to achieve (done with base graphics)

library(raster)
mex <- raster::getData(country="MEX", level=1)
munis <- raster::getData(country="MEX", level=2)
plot(munis[munis$NAME_1=="Veracruz"|munis$NAME_1=="Oaxaca",], border="green")
plot(mex[mex$NAME_1=="Veracruz"|mex$NAME_1=="Oaxaca",], add=T, lty=1)
legend("topright", c("state boundary", "munic. boundary"), pch=14, col=c("black", "green"), bty="n")

Answer 1

Here is an alternative using sf package:

# load libraries including development version of ggplot2, which is installed from GitHub (devtools::install_github('tidyverse/ggplot2'))
library(raster)
library(sf)
library(ggplot2)

mex <- raster::getData(country="MEX", level=1)
munis <- raster::getData(country="MEX", level=2) 

# convert to sf object
mex <- st_as_sf(mex) 
munis <- st_as_sf(munis) 

# subset states
mex <- mex[mex$NAME_1 %in% c("Veracruz", "Oaxaca"), ] 
munis <- munis[munis$NAME_1 %in% c("Veracruz", "Oaxaca"), ]

# plot with ggplot 
gp <- ggplot() +
  geom_sf(data = munis, aes(color = 'A'), fill = 'white', show.legend = 'line') +
  geom_sf(data = mex, aes(color = 'B'), lwd = 0.5, fill = 'transparent', show.legend = 'line') +
  scale_color_manual(values = c("A" = "green", "B" = "black"), 
                     labels = c("Municipal Boundary", "State Boundary"),
                     name = NULL) +
  theme(legend.position = c(0.75, 0.8))

# these datasets are large, so I found it is best to save to png rather than trying to load in graphics device to view
ggsave('map.png')