[GIS] Overlaying a polyshape to faceted maps in R with ggplot

cartographyggplot2overlaypolygonr

I have managed to create a faceted map from my data following the approach proposed here http://spatial.ly/2013/12/introduction-spatial-data-ggplot2/
with the following code:

ggplot(data = plot.data, aes(x = long, y = lat, fill = value, group = group)) + 
  geom_polygon() + coord_equal() + 
  facet_wrap(~variable,labeller = as_labeller(names), ncol=2)

Where 

plot.data <- merge(bound.f, mdata.melt, by.x = "id", by.y = "uid")

and bound.f is the fortified spatial polyshape (with sub-national boundaries)
and mdata.melt is the data used in map, in the the long format.

And I get the following output:

output map :In case you don't see, it is the African continent

So far so good. Now I would like to add (overlay) the boundaries of the African countries (which I have a shapefile, same projection than bound) to both maps.

In other words, I want to add the shape, which boundaries should be black and the shape with no fill.

Does anyone have a hint on how I could code this or even has a smarter approach to get beautiful maps into my publication?

Best Answer

So i finally made it ! with the following code

   map<- ggplot(data = plot.data, aes(x = long, y = lat, fill = value, group = group)) 
   + geom_polygon() + coord_equal() +
   facet_wrap(~variable,labeller = as_labeller(names))

then

map+geom_path(data=africa.f, aes(x=long, y=lat, group=group), lwd = 0.01,inherit.aes = F)

The trick was the inherit.aes = F which avoid calling on the previous layer

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