Multi-Line Highlight

This multi-line chart uses an invisible Voronoi tessellation to handle mouseover to identify the nearest point and then highlight the line on which the point falls. It is adapted from the Vega-Lite example found at https://bl.ocks.org/amitkaps/fe4238e716db53930b2f1a70d3401701

import altair as alt
from vega_datasets import data

source = data.stocks()

highlight = alt.selection(type='single', on='mouseover',
                          fields=['symbol'], nearest=True)

base = alt.Chart(source).encode(
    x='date:T',
    y='price:Q',
    color='symbol:N'
)

points = base.mark_circle().encode(
    opacity=alt.value(0)
).add_selection(
    highlight
).properties(
    width=600
)

lines = base.mark_line().encode(
    size=alt.condition(~highlight, alt.value(1), alt.value(3))
)

points + lines