Use brightest color for checkerboard when only one is available

[sgress454]

Related issue: Resolves #47128

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.
  n/a, unreleased

Testing

• Added/updated automated tests
• QA'd all new/changed functionality manually
  on main:

on branch:

For unreleased bug fixes in a release candidate, one of:

• Confirmed that the fix is not expected to adversely impact load test results

Summary by CodeRabbit

• Bug Fixes
  • Fixed checkerboard visualization color scaling to properly render non-zero data cells at the brightest color level when all non-zero values are identical.

[Copilot]

Pull request overview

Fixes Dashboard checkerboard relative scaling so that when all non-zero cells share the same value (min === max), they render at the brightest color level instead of collapsing to the dimmest level. This aligns the UI with the expected behavior in #47128 for charts like “Hosts online” when filtered down to a single host.

Changes:

• Update relative-scale color-level calculation to return level 5 when the non-zero range is 0.
• Add a regression test covering the single-distinct-value relative-scale case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
frontend/pages/DashboardPage/cards/ChartCard/CheckerboardViz.tsx	Adjusts relative-scale color selection to use brightest level when min/max are equal.
frontend/pages/DashboardPage/cards/ChartCard/CheckerboardViz.tests.tsx	Adds automated coverage to prevent regressions when relativeScale has only one distinct non-zero value.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.17%. Comparing base (b23f808) to head (88a798a).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #47129    +/-   ##
========================================
  Coverage   67.17%   67.17%            
========================================
  Files        2908     2911     +3     
  Lines      226136   226147    +11     
  Branches    11829    11683   -146     
========================================
+ Hits       151898   151906     +8     
- Misses      60508    60512     +4     
+ Partials    13730    13729     -1     

Flag	Coverage Δ
frontend	57.65% <100.00%> (+<0.01%)	⬆️

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 518ebc82-f97f-43f6-b2af-6d029f7779d6

📥 Commits

Reviewing files that changed from the base of the PR and between 2bf5106 and 88a798a.

📒 Files selected for processing (2)

• frontend/pages/DashboardPage/cards/ChartCard/CheckerboardViz.tests.tsx
• frontend/pages/DashboardPage/cards/ChartCard/CheckerboardViz.tsx

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Walkthrough

This PR fixes a bug in the CheckerboardViz component where cells with uniform non-zero values were rendered in the dimmest color instead of the brightest. The implementation removes a divide-by-zero workaround (|| 1) and adds an explicit check: when all non-zero values are identical (range = 0), all non-zero cells map to level 5 (brightest). A new test validates this behavior by verifying that zero cells remain at level 0 while all non-zero cells reach level 5 when the dataset contains only one distinct non-zero value.

Possibly related PRs

• fleetdm/fleet#45791: Both PRs modify relativeScale color-level logic in CheckerboardViz.tsx and update corresponding tests for non-zero cell coloring.
• fleetdm/fleet#44959: Both PRs adjust relativeScale min/max/ramp level computation for non-zero cells in CheckerboardViz.tsx with aligned test updates.
• fleetdm/fleet#45300: Modifies CSS color values for --level-1…--level-5 buckets in the red theme, directly connecting to the level-mapping behavior changed in this PR.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: using the brightest color for checkerboard visualization when only one color is available, which directly addresses the linked issue.
Description check	✅ Passed	The PR description includes the related issue reference (#47128), completed testing checklist items with evidence (automated tests and screenshots), and status confirmation for load testing concerns.
Linked Issues check	✅ Passed	The code changes directly address issue #47128 by implementing logic to use the brightest color when all non-zero checkerboard values are identical, with unit tests verifying this behavior.
Out of Scope Changes check	✅ Passed	All changes are scoped to the CheckerboardViz component (implementation and tests) and directly support the fix for the reported issue; no unrelated modifications detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

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• Commit unit tests in branch sgress454/bias-charts-towards-lighter-colors

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