title: how can I zoom into a map using vega-lite?
body: <p>I'm rendering some data onto a map using vega-lite. Currently, I have this schema, which renders the attached image:</p>
  
  <pre><code>{
    "title": "What's the nearest city to you?",
    "data": {
      "values": "...",
      "format": {
        "type": "csv"
      }
    },
    "encoding": {},
    "projection": {
      "type": "mercator"
    },
    "layer": [
      {
        "data": {
          "values": "...",
          "format": {
            "type": "topojson",
            "feature": "world-borders"
          }
        },
        "mark": {
          "type": "geoshape",
          "fill": "lightgrey",
          "stroke": "darkgrey"
        },
        "width": 800,
        "height": 600
      },
      {
        "mark": {
          "type": "circle",
          "size": 50
        },
        "encoding": {
          "latitude": {
            "field": "latitude",
            "type": "quantitative"
          },
          "longitude": {
            "field": "longitude",
            "type": "quantitative"
          },
          "color": {
            "field": "count",
            "type": "quantitative",
            "scale": {
              "range": [ "#f0f921", "#fcce25", "#fca636", "#f2844b", "#e16462", "#cc4778", "#b12a90", "#8f0da4", "#6a00a8", "#41049d", "#0d0887" ]
            }
          },
          "size": {
            "field": "count",
            "type": "quantitative"
          }
        }
      }
    ]
  }
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/KsuCM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KsuCM.png" alt="the rendered view of this schema"></a></p>
  
  <p>Now this looks pretty nice overall! I'm happy with it! But I would really like to be able to provide a rendered image zoomed into the densest areas of data, specifically Europe and the continental US. I've tried everything I can think of as specified in the vega and vega-lite documentation to no avail (all the properties I try have been weird—like rendering all the circles at a single pixel or just clipping the map.)</p>
  
  <p>What I'd really like is just a way to say "show me a view of data+map between lat°long° and lat°long°", but nothing jumps out at me as being designed for that purpose. How can I do this?</p>