Non-deterministic behavior with `mark_boxplot`

The following code will not order the categories on the x-axis the same way on repeated runs:

import pandas as pd
import numpy as np
import altair as alt

d = pd.DataFrame({
    'x': np.repeat(np.arange(3), 10),
    'y': np.concatenate([np.random.normal(i, 0.1, 10) for i in range(3)]),
    'z': np.repeat(np.arange(3, 0, -1), 10),
}).sort_values(
    'z',
)

alt.Chart(
    d,
).mark_boxplot(
    opacity=1.0,
).encode(
    x=alt.X('x:N', sort=None),
    y='y:Q',
    color='z:N',
)

This does not seem to happen with mark_circle.

My understanding was that using sort=None with alt.X would respect the order in the data (i.e., z).

[jakevdp]

I just switched to my mac and tried on safari, and I can see the behavior you reported (it works fine on Chrome and Firefox). It’s not an Altair issue, but rather a Vega-Lite issue (You can see it here in the vega editor).

I spent a while trying to find Safari’s developer tools to attempt to diagnose the issue, but gave up because it’s Saturday night 😄

I would report this issue on the Vega-Lite issue tracker.

[jakevdp]

As to other question about identifying a plotting library that shows plotted boxes placed mathematically instead of categorically in a box-plot, if you would like to force bins without data to be part of the x scale, in Altair you can use the scale domain argument:

subsample = source.query("age!= 25")
alt.Chart(subsample).mark_boxplot(extent='min-max').encode(
    x=alt.X('age:O', scale=alt.Scale(domain=np.arange(0, 95, 5).tolist())),
    y='people:Q'
)