Vega-Lite / Altair: How to Center or Crop a Map of Europe?

Question

I have some data on European countries. I'm trying to create a visualization in Altair / Vega-Lite using the world-110m data. Everything technically works fine, except that coded boundaries of the countries also include far-away territories, producing a terrible map that looks like this:

Here is my code:

countries = alt.topo_feature(data.world_110m.url, 'countries')
source = df.copy()

map = alt.Chart(countries).mark_geoshape(
    stroke='black'
    ).encode(
    color=alt.Color('SomeStat:Q', sort="descending", scale=alt.Scale(
        scheme='inferno', domain=(min_value,max_value)), legend=alt.Legend(title="", tickCount=6))
).transform_lookup(
    lookup='id',
    from_=alt.LookupData(source, 'CountryId', ['SomeStat', 'CountryName'])
).project(
    type='mercator'
)

Is there a way to crop this map or center it so that I'm only getting Europe and not far-flung territories all over the world?

Alternatively, is there a better public dataset I should be using that only includes Europe?

Answer 1

I don't have your df dataset, so I post rather simple example.

import altair as alt
from vega_datasets import data

countries = alt.topo_feature(data.world_110m.url, 'countries')

alt.Chart(countries).mark_geoshape(
    fill='#666666',
    stroke='white'
).project(
    type= 'mercator',
    scale= 350,                          # Magnify
    center= [20,50],                     # [lon, lat]
    clipExtent= [[0, 0], [400, 300]],    # [[left, top], [right, bottom]]
).properties(
    title='Europe (Mercator)',
    width=400, height=300
)

You can control the map view by scale and center, along with its actual plot size (width and height).

• scale: magnifying parameter
• center: center point of the view

If you need to further crop any part of the map, clipExtent can be useful. Please be careful - this array represents the pixel size, not geographical coordinates. (In the above example I have set it [[0, 0], [400, 300]] so it keeps the entire 400x300 px view.