如何在 zip 分区统计线谷歌地图的顶部添加县轮廓？

问：

我在 R 中创建了一个分区统计图。但是有人问我是否可以在地图上添加该县的轮廓。我的问题是地图必须覆盖在谷歌地图上，所以每次我尝试只添加黑色轮廓时，它都不起作用。我不太清楚我需要如何/做什么才能让它工作。

这是我的代码。注意：代码需要一个谷歌 API 密钥，它们是免费的——有限制，所以我没有包含我的 API：

#If first time using choroplethr 
#install.packages('choroplethr')
#install_github('arilamstein/[email protected]')
# Packages needed
library(tidyverse)
library(maps)
library(ggplot2)
library(ggmap)
library(mapproj)
library(ggthemes)
library(janitor)
library(devtools)
library(choroplethr)
library(choroplethrZip)
library(choroplethrMaps)

#Google API key (need to enter own unique key)
register_google(key = "")

#Dataframe
df <- structure(list(region = c("45003", "45011", "45013", "45014", 
                                "45015", "45042", "45044", "45050", "45053", "45056", "45062", 
                                "45064", "45067", "45069", "45241", "45246"), value = c(4, 103, 
                                                                                        140, 79, 30, 40, 97, 26, 5, 20, 3, 4, 32, 57, 6, 1)), row.names = c(NA, 
                                                                                                                                                            -16L), class = c("tbl_df", "tbl", "data.frame"))
#FIP (Federal Information Processing System (FIPS) Code)
fip=c(39017)

#Choropleth map
zip_choropleth(df, state_zoom = "ohio", county_zoom = fip)

missingZips <- data.frame("region" = c(45327))
missingZips$value=0

df <- rbind(df, missingZips)

get_googlemap(center = c(lon=-84.57094863369763, lat=39.45111779046858), 
              zoom=10, size = c(640, 640), scale = 2, format = c("png8"),
              maptype = c("terrain"), language = "en-EN", messaging= FALSE, 
              urlonly = FALSE, filename = NULL, color = c("bw"), force=FALSE, where = tempdir(),
              archiving = FALSE, ext = "com", inject = "")

map_blank <- zip_choropleth(df, state_zoom = "ohio", county_zoom = fip,
                           title = "title", 
                           legend= "Number of people")
map_blank

google_map <- zip_choropleth(df, state_zoom = "ohio", county_zoom = fip, reference_map = TRUE,
                              title = "title", 
                              legend= "Number of people")

google_map

#Black county outline
ohiocounty_map <-
  map_data("county") %>%
  subset(region == "ohio") %>%
  mutate(County = str_to_sentence(subregion)) %>%
  group_by(County) %>%
  filter(County %in% c("Butler"))

map_blank + geom_polygon(data=ohiocounty_map, aes(x=long, y=lat, group= group), alpha=0, 
                         color="black", size=0.2) 

#Here is where things break, I can't just add the outline to the map
google_map + geom_polygon(data=ohiocounty_map, aes(x=long, y=lat, group= group), alpha=0, 
                         color="black", size=0.2) 

这是我从最后一张地图中得到的（我被卡住了）