`sort` argument doesn't sort categories/colors in stacked bar

There are several places indicating that the argument sort can be used universally to set a custom ordering by passing an array (list) with the desired order. This works for categories on the x-axis of a grouped bar chart but not the y-axis.

Current versions:

# Name                    Version                   Build  Channel
altair                    4.0.1                      py_0    conda-forge
jupyterlab                1.2.6                      py_0    conda-forge

Example (vega-lite spec):

import pandas as pd
import altair as alt

resource_order = [
    "Onshore Wind",
    "Solar",
    "Battery",
    "NGCC",
]
resource_color_scale = alt.Scale(
    domain=resource_order, 
    range=[
        "#17becf", # onshore wind
        "#d62728",  # solar 
        '#c5b0d5', # battery
        "#bcbd22", # NGCC
    ]
)
resource_colors = alt.Color("Resource Name", scale=resource_color_scale)
    
data = pd.DataFrame(
    {
        "Resource Name": ["Battery", "NGCC", "Onshore Wind", "Solar"] * 2, 
         "Capacity": [2, 20, 10, 15, 3, 10, 30, 25], 
         "Case": ["A", "A", "A", "A", "B", "B", "B", "B"]
    }
)

alt.Chart(data).mark_bar().encode(
    x=alt.X('Case', sort=["B", "A"]), # Default order is alphabetical "A", "B"
    y=alt.Y('Capacity', sort=resource_order),
    color=resource_colors
)

I did, however, find a workaround by ordering according to a new indexing column (vega-lite spec):

resource_order_idx = {
    resource: idx 
    for idx, resource in enumerate(resource_order[::-1]) # Reverse list to align colors
}                                                        # with legend order

# Create "idx" column with integer values indicating order in stacked bar
data["idx"] = data["Resource Name"].map(resource_order_idx)

alt.Chart(data).mark_bar().encode(
    x=alt.X('Case', sort=["B", "A"]),
    y=alt.Y('Capacity'),
    order="idx",
    color=resource_colors
)

I’m not sure if this is a bug in vega-lite, an oversight in the API, or the way that it is supposed to work, but I’m providing this example and my work-around to help anyone else who might be searching for for help with the same issue.

[jakevdp]

I believe this issue tracks the functionality you want.

[jakevdp]

The order encoding is the documented way to specify sort order for stacked bars, so I believe this is working as intended.

The docs on the Altair side, as always, could be improved.