The year of data to use. Must be between 2000 and 2013.
num_colors
The number of colors on the map. A value of 1 will use a continuous scale.
A value in [2, 9] will use that many colors.
zoom
An optional vector of states to zoom in on. Elements of this vector
must exactly match the names of states as they appear in the "region" column of
?state.regions in the choroplethrMaps package.