Laura Ellis shows how to use the ggmap package to create choropleth maps in R:
In the last map, it was a bit tricky to see the density of the incidents because all the graphed points were sitting on top of each other. In this scenario, we are going to make the data all one color and we are going to set the alpha variable which will make the dots transparent. This helps display the density of points plotted.
Also note, we can re-use the base map created in the first step “p” to plot the new map.
Check it out. This is an introduction to creating choropleths, making it a good start.