Maps in R: choropleth maps by Max Marchi.
From the post:
This is the third article of the Maps in R series. After having shown how to draw a map without placing data on it and how to plot point data on a map, in this installment the creation of a choropleth map will be presented.
A choropleth map is a thematic map featuring regions colored or shaded according to the value assumed by the variable of interest in that particular region.
Another step towards becoming a map maker with R!