[R-sig-Geo] Load a map png in ggplot2 (workaround for missing world map support in ggmap)

[Markus Jenning]

Hello everybody,

my professor and I are assembling a handout on the best options for  
creating maps in R.
As it looked very up-to-date, I've been considering including a short  
tutorial on the ggmap package.

Now, as some of you might have noticed, the ggmap package does not fully  
support world maps - you can save empty maps as a png, but you can't use  
them directly in a plot, as one would usually do, e.g. with  
ggmap(get_map(...)).
Unfortunately, being able to plot data on maps of the whole world is one  
of our main priorities.

That's why I'm trying to find out how I could load pngs (or similar image  
files) into ggplot and use them as a map to plot on. I know that it's  
possible to include background images in ggplot (see e.g. this question on  
stackoverflow:  
http://stackoverflow.com/questions/16409935/plot-data-over-background-image-with-ggplot).  
However, this would only cover importing the image itself, and not  
adjusting the x/y axes etc., as it is necessary so I can plot my spatial  
data onto the map properly.

Has anyone encountered the same problem, and perhaps found a solution?
I'm looking forward to your ideas.
Please let me know if I've been unclear.

Cheers,
Markus