Lasagna Plot (Dense Time-Series Heatmap)

Method syntax

import altair as alt
from vega_datasets import data

source = data.stocks()

color_condition = (
    alt.when(alt.expr.month("datum.value") == 1, alt.expr.date("datum.value") == 1)
    .then(alt.value("black"))
    .otherwise(alt.value(None))
)

alt.Chart(source, width=300, height=100).transform_filter(
    alt.datum.symbol != "GOOG"
).mark_rect().encode(
    alt.X("yearmonthdate(date):O")
        .title("Time")
        .axis(
            format="%Y",
            labelAngle=0,
            labelOverlap=False,
            labelColor=color_condition,
            tickColor=color_condition,
        ),
    alt.Y("symbol:N").title(None),
    alt.Color("sum(price)").title("Price")
)

Attribute syntax

import altair as alt
from vega_datasets import data

source = data.stocks()

color_condition = (
    alt.when(alt.expr.month("datum.value") == 1, alt.expr.date("datum.value") == 1)
    .then(alt.value("black"))
    .otherwise(alt.value(None))
)

alt.Chart(source, width=300, height=100).transform_filter(
    alt.datum.symbol != "GOOG"
).mark_rect().encode(
    x=alt.X(
        "yearmonthdate(date):O",
        axis=alt.Axis(
            format="%Y",
            labelAngle=0,
            labelOverlap=False,
            labelColor=color_condition,
            tickColor=color_condition,
        ),
        title="Time",
    ),
    y=alt.Y("symbol:N", title=None),
    color=alt.Color("sum(price)", title="Price"),
)