title: Plotting static base map underneath a sf object
body: <p>I'm trying to plot a static base map underneath my <code>sf</code> object (for print). When using <code>ggmap</code> I first get a lot of errors, then I can't seem to figure out how to chain the base map to my <code>ggplot2</code> object with a <code>geom_sf</code>. </p>
  
  <pre><code>library(sf)
  # devtools::install_github("tidyverse/ggplot2")
  library(ggplot2)
  library(ggmap) 
  
  nc &lt;- st_read(system.file("shape/nc.shp", package="sf"))
  nc_map &lt;- get_map(location = "North Carolina, NC", zoom = 7)
  
  ggmap(nc_map)
  
  nc_centers &lt;- st_centroid(nc)
  
  nc_centers %&gt;%
    ggplot() +
    geom_sf(aes(color = SID79, size = BIR74),
            show.legend = "point") +
    coord_sf(datum = NA) +
    theme_minimal()
  </code></pre>
  
  <p>I also rather use the <code>source = "osm"</code> as style but those will always return <code>'400 Bad Request'</code>.</p>
  
  <p>Is there maybe another good package for base maps?</p>