Shading counties using FIPS code in R map - r

Question

I am looking for a way to shade counties on the US maps in R. I have list of numeric/char county FIPS code that I can input as parameter. I just need to highlight these counties -- so would just need to shade them and there are no values or variations corresponding to the counties. I tried to look up
library(choroplethr)
library(maps)
and
county_choropleth(df_pop_county)
head(df_pop_county)
region value
1 1001 54590
2 1003 183226
3 1005 27469
4 1007 22769
5 1009 57466
6 1011 10779
But these need a region, value pair. For e.g.,fips code and population in the above. Is there a way to call the county_choropleth function without having to use the values, just with the fipscode dataframe. In that way, I can my fips code with one color. What would be an efficient way to accomplish this in R using Choroplethr?

Answer 1

Here's an example using the maps library:
library(maps)
library(dplyr)
data(county.fips)
## Set up fake df_pop_county data frame
df_pop_county <- data.frame(region=county.fips$fips)
df_pop_county$value <- county.fips$fips
y <- df_pop_county$value
df_pop_county$color <- gray(y / max(y))
## merge population data with county.fips to make sure color column is
## ordered correctly.
counties <- county.fips %>% left_join(df_pop_county, by=c('fips'='region'))
map("county", fill=TRUE, col=counties$color)
Here's the resulting map:
Notice that counties with lower FIPS are darker, while counties with higher FIPS are lighter.