County   State   Value_x
.
Adair    Oklahoma  5
Adair    Missouri  2
Adair    Kentucky  10
Adair    lowas     1
.

library(raster)
library(leaflet)
library(tidyverse)

# Get USA polygon data
USA <- getData("GADM", country = "usa", level = 2)

### Get data. I can't provide the date set. It has county, state and value _x as shown above
mydata <- county,state,value_x

### Check counties that exist in USA, but not in mydata
### Create a dummy data frame and bind it with mydata

mydata <- data.frame(COUNTY_NAME = setdiff(USA$NAME_2, mydata$county),
                     value_x = NA,
                     stringsAsFactors = FALSE) %>%
          bind_rows(mydata)

### Create a color palette
mypal <- colorNumeric(palette = "viridis", domain = mydata$value_x)

leaflet() %>%
addProviderTiles("OpenStreetMap.Mapnik") %>%
setView(lat = 39.8283, lng = -98.5795, zoom = 4) %>%
addPolygons(data = USA, stroke = FALSE, smoothFactor = 0.2, fillOpacity = 0.3,
            fillColor = ~mypal(mydata$value_x),
            popup = paste("Region: ", USA$NAME_2, "<br>",
                          "Value X: ", mydata$value_x, "<br>")) %>%
 addLegend(position = "bottomleft", pal = mypal, values = mydata$value_x,
           title = "Value X",
           opacity = 1)