In ggplot2, it's easy to create a faceted plot with facets that span both rows and columns. Is there a "slick" way to do this in altair? facet documentation

It's possible to have facets plot in a single column,

import altair as alt
from vega_datasets import data
iris = data.iris

chart = alt.Chart(iris).mark_point().encode(
    x='petalLength:Q',
    y='petalWidth:Q',
    color='species:N'
).properties(
    width=180,
    height=180
).facet(
    row='species:N'
)

and in a single row,

chart = alt.Chart(iris).mark_point().encode(
    x='petalLength:Q',
    y='petalWidth:Q',
    color='species:N'
).properties(
    width=180,
    height=180
).facet(
    column='species:N'
)

but often, I just want to plot them in a grid using more than one column/row, i.e. those that line up in a single column/row don't mean anything in particular.

For example, see facet_wrap from ggplot2: http://www.cookbook-r.com/Graphs/Facets_(ggplot2)/#facetwrap

In Altair version 3.1 or newer (released June 2019), wrapped facets are supported directly within the Altair API. Modifying your iris example, you can wrap your facets at two columns like this:

import altair as alt
from vega_datasets import data
iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalLength:Q',
    y='petalWidth:Q',
    color='species:N'
).properties(
    width=180,
    height=180
).facet(
    facet='species:N',
    columns=2
)

Alternatively, the same chart can be specified with the facet as an encoding:

alt.Chart(iris).mark_point().encode(
    x='petalLength:Q',
    y='petalWidth:Q',
    color='species:N',
    facet=alt.Facet('species:N', columns=2)
).properties(
    width=180,
    height=180,
)

The columns argument can be similarly specified for concatenated charts in alt.concat() and repeated charts alt.Chart.repeat().