fix: remove invalid stack property from secondary position channels#237
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thomasp85 merged 1 commit intoposit-dev:mainfrom Mar 18, 2026
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`build_layer_encoding` was emitting `"stack": null` on both primary (y/radius) and secondary (y2/radius2) position channels. The Vega-Lite v6 schema does not allow `stack` on secondary channels, causing Altair to reject the spec with "Only chart objects can be used in LayerChart" (validate=False) or a schema validation error (validate=True). Only primary position channels need `stack: null` to disable Vega-Lite's automatic stacking — secondary channels inherit from the primary. Also strengthens `test_secondary_channels_have_no_disallowed_properties` to assert `stack` is absent and to run full Vega-Lite schema validation. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Problem
build_layer_encodingemits"stack": nullon both primary (y/radius) and secondary (y2/radius2) position channels to disable Vega-Lite's automatic stacking. However, the Vega-Lite v6 schema does not allowstackon secondary channels — only primary position channels support it.This causes errors when consuming the JSON output with Altair (Python):
validate=False:TypeError: Only chart objects can be used in LayerChart.validate=True:ValidationError: Additional properties are not allowed ('stack' was unexpected)Any spec that produces a
y2encoding (bars, areas, segments, ribbons, error bars, etc.) is affected.Fix
Remove the
stack: nullinsertion ony2/radius2channels. Only primary position channels (y/radius) need this property — secondary channels don't support independent stacking and inherit behavior from their primary channel.Test changes
Strengthened
test_secondary_channels_have_no_disallowed_propertiesto:stackis absent fromx2/y2encodings (was not previously checked)assert_valid_vegalite()for full Vega-Lite schema validation (was not previously called)