Relates to #313. For layered and multi-view charts, Altair takes a bit of a shortcut and does not use the exact classes which map to the Vega-Lite schema but instead uses the API convenience classes such as alt.Chart, alt.LayerChart, etc. For example for the layered chart below, the elements of layer are alt.Chart instances:

import altair as alt
import vegafusion as vf
from vega_datasets import data

source = data.wheat()

bar = alt.Chart(source).mark_bar().encode(
    x='year:O',
    y='wheat:Q'
)

rule = alt.Chart(source).mark_rule(color='red').encode(
    y='mean(wheat):Q'
)

layer_chart = (bar + rule).properties(width=600)
type(layer_chart.layer[0])  # altair.vegalite.v5.api.Chart

However, when reconstructing this chart from a dictionary, the layers are of type UnitSpec which is the correct class following the VL schema:

reconstructed_layer_chart = alt.Chart.from_dict(layer_chart.to_dict())
type(reconstructed_layer_chart.layer[0])  # altair.vegalite.v5.schema.core.UnitSpec

When calling transformed_data on the reconstructed chart, it fails as the isinstance checks do not accept UnitSpec as an equivalent to alt.Chart although it should be:

vf.transformed_data(reconstructed_layer_chart)

ValueError: transformed_data accepts an instance of Chart, FacetChart, LayerChart, HConcatChart, VConcatChart, or ConcatChart
Received value of type: <class 'altair.vegalite.v5.schema.core.UnitSpec'>

All classes which I think can be treated as equivalent to the first one mentioned in every bullet:

• Chart: TopLevelUnitSpec (parent class of Chart), FacetedUnitSpec, UnitSpec, UnitSpecWithFrame, NonNormalizedSpec (used in concat charts for chart objects in e.g. hconcat attribute)
• LayerChart: TopLevelLayerSpec (parent class of LayerChart), LayerSpec
• RepeatChart: TopLevelRepeatSpec (parent class of RepeatChart), RepeatSpec
• ConcatChart: TopLevelConcatSpec (parent class of ConcatChart), ConcatSpecGenericSpec
• HConcatChart: TopLevelHConcatSpec (parent class of HConcatChart), HConcatSpecGenericSpec
• VConcatChart: TopLevelVConcatSpec (parent class of VConcatChart), VConcatSpecGenericSpec
• FacetChart: TopLevelFacetSpec (parent class of FacetChart), FacetSpec

Spec classes which I'm not sure about but we can probably ignore as repeat charts are not supported yet by transformed_data anyway:

• LayerRepeatSpec
• NonLayerRepeatSpec
• GenericUnitSpecEncodingAnyMark

Just a reminder that the same changes would need to be applied to https://github.com/altair-viz/altair/blob/master/altair/utils/_transformed_data.py.

Btw, my use case for the above is that a frontend sends the chart specifications as a JSON to a backend API which executes vf.transformed_data(alt.Chart.from_dict(spec)) and then returns the extracted data as an Excel file to the user of the web application.