How can I facet on multiple variables?

[rongcuid]

I tried to figure out by reading examples/docs of both Altair and Vega-Lite, but I don't find any. What I am trying to achieve is the following equivalent in ggplot2:

facet_wrap(["V1", "V2", "V3"])

However, I can't find anyway to make Altair .facet accept multiple fields. There seem to be some mention about nested facets in Vega-Lite Github issues, but I can't find if it's actually implemented or how to use them.

Any suggestions?

[joelostblom]

You could use row and column like in this example https://altair-viz.github.io/gallery/bar_chart_faceted_compact.html. If you would like to include more variables than two, I think you would have to create new combined columns of the variable combinations first. But I'm not 100% on how ggplot handles it, so if this does not sound like what you mean, feel free to include a short example with complete ggplot code and output.

[rongcuid]

I am specifically trying to facet with more than 2 variables.

Example using Plotnine:

# %%
from plotnine import *
from plotnine.data import mpg

(
    ggplot(mpg, aes("cty", "hwy"))
    + geom_point(size=.5)
    + facet_wrap(["manufacturer", "model", "drv"])
    + theme(figure_size=(6, 6))
)

[joelostblom]

There is currently no support for that in Vega-Lite so it is not available in Altair. Instead, you can create the new combined column with the dataframe library you're using, or directly in altair using transform_calculate:

import pandas as pd


mpg = pd.read_csv('https://raw.githubusercontent.com/has2k1/plotnine/4159f9548fa5f95c02366efc324e88f9ba79af09/plotnine/data/mpg.csv')

alt.Chart(mpg, width=100, height=100).mark_point().transform_calculate(
    man_model_drv=alt.expr.slice([alt.datum.manufacturer, alt.datum.model, alt.datum.drv])
    # Equivalent JS string expression
    # man_model_drv='[datum.manufacturer, datum.model, datum.drv]'
).encode(
    x='cty',
    y='hwy',
).facet(
    'man_model_drv:N',
    columns=7
)

---

Initial suggestion before switching to creating the array directly:

    # the call to `split` is to create the newline in the title
    man_model_drv=alt.expr.split(alt.datum.manufacturer + "-" + alt.datum.model + "-" + alt.datum.drv, '-')

[dangotbanned]

@joelostblom do we have an example like this in the gallery?

I'm somewhat blown away by how quickly you recreated #3601 (reply in thread)

[joelostblom]

Not yet, but I could maybe append on to https://altair-viz.github.io/user_guide/compound_charts.html#faceted-charts? I was planning to update that section with some other changes anyways.

[rongcuid]

Can I just join them with \n or something? Some of my variables have - in it, and I would like to avoid picking some special character every time.

[joelostblom]

You can join with any character that split accepts, since you need an array for text to render as multi line. I'm not sure on the top of my head if newline is supported, but try it it out.