theme_map: This creates a nice map theme for use in plot_usmap.
It is borrowed from the ggthemes package located at this repository:
https://github.com/jrnold/ggthemes
Description
This function was manually rewritten here to avoid the need for
another package import.
Usage
theme_map(base_size = 9, base_family = "")
Details
All theme functions (i.e. theme_bw, theme, element_blank,
come from ggplot2.