How to make vertical line in altair

vertical-line-altair.py

from vega_datasets import data as vega_data
import pandas as pd
import altair as alt

data = pd.read_json(vega_data.gapminder.url)
data2000 = data.loc[data['year'] == 2000]

chart1 = alt.Chart(data2000).mark_line().encode(
    alt.X('fertility:Q'),
    alt.Y('life_expect:Q'))

### fake points for vertical line ###
vertical_line = alt.Data(values=[{'y_start':0,
                                  'y_end':60,
                                  'x_pos':6}])

chart2 = alt.Chart(vertical_line).mark_line().encode(
    alt.Y('y_start:Q'),
    alt.Y2('y_end:Q'),
    alt.X('x_pos:Q'))

chart1 + chart2

will produce a chart that looks like this

cc: @dansbecker