title: R: State and County Outlines for choroplethrZip
body: <p>I have a dataset with zip codes that I have used choroplethrZip to graph the data. I am looking at the data on the state and county level.  However, zip codes don't necessarily correspond with state and county lines. I have tried to use reference_map = TRUE, but it doesn't have county lines, and starts to look a little busy.  Is there a way to change the default reference map to one that has state and county lines without the rest of the map detail? In other words, I don't want streets and topography.  </p>
  
  <p>Here is my code with the sample data that is similar to the data I am using.  You can see the issue with the state of Texas boundaries. </p>
  
  <pre><code>#zip.regions metadata file for choroplethrZip
  data(zip.regions)
  head(zip.regions)
  
  #Test data file:A data.frame containing population estimates 
  # for US Zip Code Tabulated Areas (ZCTAs) in 2012.
  data(df_pop_zip) 
  
  #Create a choropleth of US Zip Codes
  zip_choropleth(df_pop_zip, 
                 state_zoom="texas", 
                 title="2012 Texas State ZCTA Population Estimates",
                 legend="Population",
                 reference_map = TRUE)
  
  #Zoom County
  dd_fips = c(48113, 48121)
  zip_choropleth(df_pop_zip, 
                 county_zoom=dd_fips,  
                 title="2012 Denton &amp; Dallas ZCTA Population Estimates",
                 legend="Population",
                 reference_map = TRUE)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/LTgAP.png" rel="nofollow noreferrer">TexasPlot</a></p>
  
  <p><a href="https://i.stack.imgur.com/Gc05B.png" rel="nofollow noreferrer">DentonDallasPlot</a></p>