sort channel not working when I add text on top of my bars

This sorts like I want.

import altair as alt
from vega_datasets import data

barley = data.barley()
alt.Chart(cars.head(10)).mark_bar().encode(
    x='Miles_per_Gallon:Q',
    y=alt.Y('Name:N', sort=alt.SortField(field="Miles_per_Gallon", op="sum", order="descending"))
)

This does not.

bars = alt.Chart(cars.head(10)).mark_bar().encode(
    x='Miles_per_Gallon:Q',
    y=alt.Y('Name:N', sort=alt.SortField(field="Miles_per_Gallon", op="sum", order="descending"))
)

text = bars.mark_text(
    align='left',
    baseline='middle',
    dx=3,
).encode(text='Miles_per_Gallon:Q')

bars + text

Why?

[jakevdp]

I’m not sure why this is happening; I think it’s a vega-lite bug. Here’s a simplified vega-lite spec showing the issue (Open the Chart in the Vega Editor):

{
  "data": {"url": "https://vega.github.io/vega-datasets/data/barley.json"},
  "transform": [
    {
      "joinaggregate": [{"op": "sum", "field": "yield", "as": "order"}],
      "groupby": ["variety"]
    }
  ],
  "encoding": {
    "x": {
      "field": "yield",
      "aggregate": "sum",
      "type": "quantitative",
      "stack": "zero"
    },
    "y": {
      "field": "variety",
      "type": "nominal",
      "sort": {"field": "order", "order": "descending"}
    },
    "order": {"type": "nominal", "field": "site"}
  },
  "layer": [
    {
      "mark": "bar",
      "encoding": {"color": {"type": "nominal", "field": "site"}}
    },
    {
      "transform": [{"filter": "true"}],
      "mark": {"type": "text", "color": "white", "dx": -15, "dy": 3},
      "encoding": {
        "text": {
          "type": "quantitative",
          "aggregate": "sum",
          "field": "yield",
          "format": ".1f"
        }
      }
    }
  ],
  "width": 600
}

If you delete the filter line, the sort works as expected.

I think it’s probably some issue with unioned domains, though strangely there is no warning in the Vega editor.

As a workaround, you can set resolve_scale(y='independent'), which preserves the sort:

import altair as alt
from vega_datasets import data

source=data.barley.url

bars = alt.Chart(source).transform_joinaggregate(
    order='sum(yield)', groupby=['variety']  
).mark_bar().encode(
    x=alt.X('sum(yield):Q', stack='zero'),
    y=alt.Y('variety:N', sort=alt.SortField('order', order='descending')),
    color=alt.Color('site:N'),
    order = 'site:N'
)

text = bars.mark_text(
    dx=-15, dy=3,
).transform_filter(
    {'field': 'site', 'oneOf': ['Crookston', 'Duluth']}
).encode(
    color=alt.value('white'),
    text=alt.Text('sum(yield):Q', format='.1f'),
    y=alt.Y('variety:N', sort=alt.SortField('order', order='descending'), axis=None),
)

(bars + text).resolve_scale(y='independent')

[jakevdp]

The default aggregation is set within Vega-Lite. It might be worth a feature request there.