Parallel coordinates in Altair

Question

I want to do a parallel coordinates plot with multiple y axis. I've found how to do it in Vega-Lite here but I haven't found the way to do it with Altair, there's only a very simple example where all the y axis are the same. Is there any way to do this plot in altair?

Answer 1

Note that this kind of chart is not "built-in" to Altair or Vega-Lite, so the only way to create it is with a manual sequence of transforms, and manually constructing your axes from tick and text marks.

Here is an Altair version of the chart in the answer you linked to:

import altair as alt
from vega_datasets import data

base = alt.Chart(
    data.iris.url
).transform_window(
    index="count()"
).transform_fold(
    ["petalLength", "petalWidth", "sepalLength", "sepalWidth"]
).transform_joinaggregate(
    min="min(value)",
    max="max(value)",
    groupby=["key"]
).transform_calculate(
    norm_val="(datum.value - datum.min) / (datum.max - datum.min)",
    mid="(datum.min + datum.max) / 2"
).properties(width=600, height=300)

lines = base.mark_line(opacity=0.3).encode(
    x='key:N',
    y=alt.Y('norm_val:Q', axis=None),
    color="species:N",
    detail="index:N",
    tooltip=["petalLength:N", "petalWidth:N", "sepalLength:N", "sepalWidth:N"]
)

rules = base.mark_rule(
    color="#ccc", tooltip=None
).encode(
    x="key:N",
    detail="count():Q",
)

def ytick(yvalue, field):
    scale = base.encode(x='key:N', y=alt.value(yvalue), text=f"min({field}):Q")
    return alt.layer(
        scale.mark_text(baseline="middle", align="right", dx=-5, tooltip=None),
        scale.mark_tick(size=8, color="#ccc", orient="horizontal", tooltip=None)
    )

alt.layer(
    lines, rules, ytick(0, "max"), ytick(150, "mid"), ytick(300, "min")
).configure_axisX(
    domain=False, labelAngle=0, tickColor="#ccc", title=None
).configure_view(
    stroke=None
)