title: Include map scale and north arrow to only one ggplot facet
body: <p>I am ploting a map using the <code>ggplot2</code> and <code>sf</code> packages and I would like to include a map scale and north arrow to the figure using the <a href="https://github.com/oswaldosantos/ggsn" rel="nofollow noreferrer"><code>ggsn</code></a> package. Here is the problem. I want to show a figure with two or more <code>facets</code> but I would like to add the scale and north arrow to only one of the figure facets.</p>
  
  <p>Any idea on how to do this?</p>
  
  <p><strong>Reproducible example:</strong></p>
  
  <pre><code>library(UScensus2000tract)
  library(ggplot2)
  library(spdep)
  library(sf)
  library(ggsn)
  
  # load data
    data("oregon.tract")
  
  # convert spatial object to sf class
    oregon_sf_A &lt;- st_as_sf(oregon.tract)
    oregon_sf_B &lt;- st_as_sf(oregon.tract)
  
  # organize data for facetting
    oregon_sf_A$cat &lt;- "A"
    oregon_sf_B$cat &lt;- "B"
    oregon_sf &lt;- rbind(oregon_sf_A, oregon_sf_B)
  
  # create a second variable for facetting
    oregon_sf$var2 &lt;- sample(c("C","D"), nrow(oregon_sf), replace=T)
  
  # Map
    ggplot() +
      geom_sf(data=oregon_sf, aes(fill=hispanic.t), color=NA) +
      scale_fill_viridis(option = "inferno", direction=-1) +
      facet_grid(~cat) +
      north(data=oregon_sf, symbol=4, location="bottomleft", scale=.2) +
      scalebar(data=oregon_sf, dist = 200, st.size=3, height=0.02, dd2km = TRUE, location="bottomright" , model = 'WGS84')
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/2n7Ug.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2n7Ug.png" alt="enter image description here"></a></p>
  
  <p>The problem gets a bit more complicated if we try to use two variables in <code>facet_grid</code>:</p>
  
  <pre><code># Map 2
    ggplot() +
      geom_sf(data=oregon_sf, aes(fill=hispanic.t), color=NA) +
      scale_fill_viridis(option = "inferno", direction=-1) +
      facet_grid(cat~var2) +
      north(data=oregon_sf, symbol=4, location="bottomleft", scale=.2) +
      scalebar(data=oregon_sf, dist = 200, st.size=3, height=0.02, dd2km = TRUE, location="bottomright" , model = 'WGS84')
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/oD24J.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/oD24J.png" alt="enter image description here"></a></p>