R - Getting correct two color gradient on ggplot2 map

Question

I'm trying to plot a gradient fill map of all counties in the United States using the following code. I'd like to have a continuous darkblue -> darkred gradient fill for the map fill, but for some reason my code isn't doing this even though I added:

scale_colour_gradientn("Title", colours = c("darkblue", "darkred"),
                        values = c(0,1))

to the plot. How do I make a gradual gradient from darkblue to darkred in this case (data scale is from 0 to 1)?

counties_dem_rep <- read.csv("2016_US_County_Level_Presidential_Results.csv", header = TRUE, sep = ",")

library(ggplot2)
library(fiftystater)
library(colorplaner)
library(USAboundaries)
library(sf)
library(dplyr)

usc <- us_counties(resolution = c("low", "high"), states = NULL)

temp_count_NM <- us_counties(states = "New Mexico", resolution ='high')

st_list = c("Alabama", "Arizona", "Arkansas", "California", "Colorado", 
            "Connecticut", "Delaware", "Florida", "Georgia", "Idaho", "Illinois", 
            "Indiana", "Iowa", "Kansas", "Kentucky", "Louisiana", "Maine", "Maryland", 
            "Massachusetts", "Michigan", "Minnesota", "Mississippi", "Missouri", 
            "Montana", "Nebraska", "Nevada", "New Hampshire", "New Jersey", "New 
            Mexico", "New York", "North Carolina", "North Dakota", "Ohio", "Oklahoma", 
            "Oregon", "Pennsylvania", "Rhode Island", "South Carolina", "South Dakota", 
            "Tennessee", "Texas", "Utah", "Vermont", "Virginia", "Washington", "West 
            Virginia", "Wisconsin", "Wyoming")

plot_counties <- us_counties(states=st_list, resolution = 'high')
plot_counties$geoid <- as.numeric(plot_counties$geoid)
colnames(counties_dem_rep)[11] <- "geoid"

plot_counties <- inner_join(plot_counties, counties_dem_rep, by = "geoid")

plot_states <- us_states(states=st_list, resolution = 'high')

temp_count_NM$geoid <- as.numeric(temp_count_NM$geoid)

temp_count_NM <- inner_join(temp_count_NM, counties_dem_rep, by = "geoid")



devtools::install_github("tidyverse/ggplot2")
require(ggplot2)

ggplot(plot_counties) + 
  geom_sf(aes(fill=per_gop), color=NA) + 
  geom_sf(data=plot_states, fill=NA, color="black", lwd=0.25) + 
  scale_x_continuous(breaks = NULL) + 
  scale_y_continuous(breaks = NULL) +
  scale_colour_gradientn("Title", colours = c("darkblue", "darkred"),
                        values = c(0,1))+ 
  theme(panel.background = element_blank(), axis.ticks = element_blank(), 
        axis.text = element_blank())

At the moment I get the following plot:

Answer 1

You've set your fill to a value and color to NA in this line geom_sf(aes(fill=per_gop), color=NA) . I suspect this means you should be using scale_fill_gradientn not scale_color_gradientn .