spec.json

{"$schema": "https://vega.github.io/schema/vega/v3.0.json",
  "width": 500,
  "height": 600,
  "autosize": "none",
  "signals": [
    {
      "name": "translate0",
      "update": "width / 2"
    },
    {
      "name": "translate1",
      "update": "height / 2"
    }
  ],
  "projections": [
    {
      "name": "projection",
      "type": "mercator",
      "scale": 1000,
      "rotate": [
        0,
        0,
        0
      ],
      "center": [
        17,
        -3
      ], 
      "translate": [
        {
          "signal": "translate0"
        },
        {
          "signal": "translate1"
        }
      ]

    }
  ],
  "data": [
    {
      "name": "drc",
      "url": "https://gist.githubusercontent.com/thomas-maschler/ef9891ef03ed4cf3fb23a4378dab485e/raw/47f3632d2135b9a783eeb76d0091762b70677c0d/drc.geojson",
      "format": {
        "type": "json",
        "property": "features"
      }
    }
  ],
  "marks": [
    {
      "type": "shape",
      "from": {
        "data": "drc"
      },
      "encode": {
        "update": {
          "strokeWidth": {
            "value": 0.5
          },
          "stroke": { 
            "value": "darkblue"
          },
        
          "fillOpacity": {
            "value": 0.5
          }
        }
       
      },
      "transform": [
        {
          "type": "geoshape",
          "projection": "projection"
        }
      ]
    }
  ]
}