line_df = gapminder.query(" country == 'Malawi' ")

Original plot

Line Chart

(ggplot(line_df, aes(x='year', y='lifeExp')) + bbc_style() +
 geom_line(colour='#1380A1', size=1) +
 geom_hline(yintercept=0, size=1, colour='#333333') +
 labs(title='Living longer',
      subtitle='Life expectancy in Malawi 1952-2007')
 )

<ggplot: (-9223371893446801452)>

# altair
line = (alt.Chart(line_df).mark_line().encode(
    x=alt.X('year', axis=alt.Axis(values=list(range(1950, 2020, 10)))),
    y=alt.Y('lifeExp', axis=alt.Axis(values=list(range(0, 60, 10)))))
    .properties(title={'text': 'Living Longer',
                       'subtitle': ['Life expectancy in Malawi 1952-2007'],
                       }, )
)

# hline
overlay = pd.DataFrame({'lifeExp': [0]})
hline = (
    alt.Chart(overlay)
    .mark_rule(color='#333333', strokeWidth=3)
    .encode(y='lifeExp')
)

(line + hline).configure_axisX(grid=False)