A great and easier package to work with for creating maps using pure R is ggmaps: http://journal.r-project.org/archive/2013-1/kahle-wickham.pd...
Here's an example map of San Francisco I made using ggmaps: http://i.imgur.com/zdetaKw.png
(The map was made for http://minimaxir.com/2014/04/san-francisco/ if you want more context.)