Fix KeyError when sorting melted bar chart data#13695
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lukasmasuch merged 2 commits intodevelopfrom Jan 26, 2026
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Pull request overview
This PR fixes a KeyError that occurred when sorting bar charts by the x column while plotting multiple y columns, and adds regression coverage for this behavior. It also updates bar chart E2E tests and snapshots to reflect the new chart case.
Changes:
- Updated
_maybe_meltinbuilt_in_chart_utilsto avoid duplicate id columns when melting data, preventing pandasKeyErrorduring chart preparation. - Added a Python unit test in
vega_charts_test.pyto verify descending sort by the x column when multiple y columns are specified, and to guard against regressions of theKeyError. - Extended the bar chart E2E app and Playwright tests to include a new sort scenario (
xwith multipleycolumns) and adjusted chart counts and indices accordingly.
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Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
lib/streamlit/elements/lib/built_in_chart_utils.py |
Fixes the melt logic by ensuring size_column and sort_column are not added twice to columns_to_leave_alone, eliminating duplicate id vars that caused KeyError in pandas.melt. |
lib/tests/streamlit/elements/vega_charts_test.py |
Adds a regression unit test confirming that sorting by the x column with multiple y columns no longer raises KeyError and that descending sort on the x axis is correctly encoded. |
e2e_playwright/st_bar_chart.py |
Adds a new st.bar_chart example that sorts by the x column while using multiple y columns to exercise the fixed behavior end-to-end. |
e2e_playwright/st_bar_chart_test.py |
Updates total chart count, snapshot indices, and adds a new snapshot for the multi-y x-sort case, plus shifts the add-rows chart index to remain aligned with the app. |
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✅ Snyk checks have passed. No issues have been found so far.
💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse. |
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📉 Frontend coverage change detectedThe frontend unit test (vitest) coverage has decreased by 0.0500%
💡 Consider adding more unit tests to maintain or improve coverage. |
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Describe your changes
Fix KeyError when using st.bar_chart() with sort parameter set to the same column as x while specifying multiple y columns.
Root cause: In _maybe_melt(), when building columns_to_leave_alone for pandas' melt operation, the code added x_column, size_column, and sort_column without checking for duplicates. When x_column == sort_column (e.g., x="a", sort="-a"), the list contained duplicates like ["a", "a"]. Pandas' melt tried to pop the column twice, succeeding the first time but failing with KeyError on the second attempt.
Fix: Added duplicate checks before appending size_column and sort_column to columns_to_leave_alone.
Testing Plan
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By submitting this pull request you agree that all contributions to this project are made under the Apache 2.0 license.