Altair - Setting a custom date domain for the x axis

blog_domain.py

# Code for https://markhneedham.com/blog/2020/01/14/altair-range-values-dates-axis/ blog post

import altair as alt
import pandas as pd
import datetime

df = pd.DataFrame( [
    {"position": 40, "date": datetime.date(2019,9,5)},
    {"position": 31, "date": datetime.date(2019,9,12)},
    {"position": 19, "date": datetime.date(2019,9,19)},
    {"position": 14, "date": datetime.date(2019,9,26)},
    {"position": 7, "date": datetime.date(2019,10,3)},
    {"position": 1, "date": datetime.date(2019,10,10)},
    {"position": 1, "date": datetime.date(2019,10,17)},
    {"position": 1, "date": datetime.date(2019,10,24)},
])

df["date"] = pd.to_datetime(df["date"])

chart = alt.Chart(df).mark_point(color="red").encode(
    x='date', 
    y='position')

chart.save("chart.html")

# domain is Pandas index
domain_pd = pd.to_datetime(['2019-01-01', '2019-12-31']).astype(int) / 10 ** 6
chart = alt.Chart(df).mark_point(color="red").encode(
    x=alt.X('date:T', timeUnit='yearmonthdate', scale=alt.Scale(domain=list(domain_pd))),
    y='position')
chart.save("chart.html")

# domain is start and end dates
domain = ["2019-01-01", "2019-12-31"]
chart = alt.Chart(df).mark_point(color="red").encode(
    x=alt.X('date:T', timeUnit='yearmonthdate', scale=alt.Scale(domain=domain)),
    y='position')
chart.save("chart.html")