Choropleth Maps in ggplot2

How to make Choropleth Maps in ggplot2 with Plotly.

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County-Level Boundaries

library(plotly)
library(maps)

county_df <- map_data("county")
state_df <- map_data("state")

# create state boundaries
p <- ggplot(county_df, aes(long, lat, group = group)) +
  geom_polygon(colour = alpha("black", 1/2), fill = NA) +
  geom_polygon(data = state_df, colour = "black", fill = NA) + 
  theme_void()

fig <- ggplotly(p)

fig