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
This function was manually rewritten here to avoid the need for another package import.
theme_map(base_size = 9, base_family = "")
All theme functions (i.e. theme_bw, theme, element_blank, come from ggplot2.
Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.