title: How can I make a map using GeoJSON data in Altair?
body: <p>I'm very new to mapping, and to Altair/Vega. There's <a href="https://altair-viz.github.io/gallery/airports_count.html" rel="noreferrer">an example in the Altair documentation for how to make a map starting with an outline of US states</a>, which is created basically with: </p>
  
  <pre><code>states = alt.topo_feature(data.us_10m.url, feature='states')
  
  # US states background
  background = alt.Chart(states).mark_geoshape(
      fill='lightgray',
      stroke='white'
  )
  </code></pre>
  
  <p>but I want to plot points in the British Isles, instead. Since there are only US and World maps in the vega data collections, I would have to create my own GeoJSON, no? </p>
  
  <p>So I tried getting GeoJSON for the British Isles from a world map, by running some of the command-line commands <a href="https://www.vicchi.org/2013/02/27/the-great-british-map-or-great-britain-vs-the-united-kingdom-vs-the-british-isles/" rel="noreferrer">from this blog post</a>, namely, </p>
  
  <pre><code>ogr2ogr -f GeoJSON -where "adm0_a3 IN ('GBR','IRL','IMN','GGY','JEY','GBA')" subunits.json ne_10m_admin_0_map_subunits/ne_10m_admin_0_map_subunits.shp
  </code></pre>
  
  <p>This seems to have created a GeoJSON file, subunits.json, which probably represents the British Isles. But how can I get this into Altair? Or is there another way to make a map of the British Isles using Altair? </p>