How to create a TopoJSON geomap using Altair library for Python?

Question

I am trying to create a geomap of a state using Altair package for Python. I will then plot let's say dots representing some event at a location specified by its latitude & longitude. I follow the Altair's example gallery here. The TopoJSON file is located in the GitHub repository deldersveld/topojson. However, I can't get it to draw the map of Michigan. Is this file missing something? Anyone who can help me?

Environment:

• Windows 10
• python 3
• jupyter-lab 1.0.0a3
• altair 3.0

Answer 1

You can do something like this:

import altair as alt

url = "https://raw.githubusercontent.com/deldersveld/topojson/master/countries/us-states/MI-26-michigan-counties.json"

source = alt.topo_feature(url, "cb_2015_michigan_county_20m")

alt.Chart(source).mark_geoshape().encode(
    tooltip='properties.NAME:N'
)

The key is to look in the "objects" and "properties" entries of the TopoJSON file to figure out what to use for features and encodings.