Line Chart with Datum

An example of using datum to highlight certain values, including a DateTime value. This is adapted from two corresponding Vega-Lite Examples: Highlight a Specific Value.

Method syntax

import altair as alt
from vega_datasets import data

source = data.stocks()

lines = (
    alt.Chart(source)
    .mark_line()
    .encode(x="date", y="price", color="symbol")
)

xrule = (
    alt.Chart()
    .mark_rule(color="cyan", strokeWidth=2)
    .encode(x=alt.datum(alt.DateTime(year=2006, month="November")))
)

yrule = (
    alt.Chart().mark_rule(strokeDash=[12, 6], size=2).encode(y=alt.datum(350))
)


lines + yrule + xrule

Attribute syntax

import altair as alt
from vega_datasets import data

source = data.stocks()

lines = (
    alt.Chart(source)
    .mark_line()
    .encode(x="date", y="price", color="symbol")
)

xrule = (
    alt.Chart()
    .mark_rule(color="cyan", strokeWidth=2)
    .encode(x=alt.datum(alt.DateTime(year=2006, month="November")))
)

yrule = (
    alt.Chart().mark_rule(strokeDash=[12, 6], size=2).encode(y=alt.datum(350))
)


lines + yrule + xrule
# No channel encoding options are specified in this chart
# so the code is the same as for the method-based syntax.