Tile Choropleth — MapLibre

Mix.install([
  {:plotly_ex, "~> 0.3"},
  {:kino, "~> 0.18"}
])

Embedded GeoJSON regions

Choroplethmap uses layout.map and token-free tiles. This small synthetic GeoJSON avoids downloading a county dataset; production county boundaries use the same featureidkey and locations mapping. Transparency belongs inside marker, not at the trace's top level.

alias Plotly.{Figure, Choroplethmap}

geojson = %{
  type: "FeatureCollection",
  features: [
    %{type: "Feature", id: "west", properties: %{}, geometry: %{
      type: "Polygon", coordinates: [[[-74.04, 40.70], [-74.00, 40.70],
        [-74.00, 40.75], [-74.04, 40.75], [-74.04, 40.70]]]
    }},
    %{type: "Feature", id: "east", properties: %{}, geometry: %{
      type: "Polygon", coordinates: [[[-74.00, 40.70], [-73.96, 40.70],
        [-73.96, 40.75], [-74.00, 40.75], [-74.00, 40.70]]]
    }}
  ]
}

Figure.new()
|> Figure.add_trace(Choroplethmap.new(
  geojson: geojson, locations: ["west", "east"], z: [12, 28],
  featureidkey: "id", colorscale: "YlOrRd", marker: %{opacity: 0.65}
))
|> Figure.update_layout(
  title: "Two example regions",
  map: %{style: "carto-positron", center: %{lat: 40.725, lon: -74.0}, zoom: 11}
)
|> Plotly.show()