CODAP-1461: don't show an LSRL equation when no line can be fit

[kswenson]

Fixes CODAP-1461

Problem

With a scatter plot whose points are all identical (or vertically aligned) and
the Least Squares Line enabled, the graph showed a bogus equation
x = (not a number) with no line drawn.

Root cause

leastSquaresLinearRegression computes slope = sumOfProductDiffs / xSumSquaredDeviations. When the x values don't vary that is 0/0, so slope,
intercept and r² all come back NaN. The LSRL adornment stores those NaNs, and
its guards test != null — which NaN passes — so the equation string fell into
its vertical-line branch and rendered x = (not a number). The line itself never
appeared because the endpoint code does check for finite values.

Fix

Compute the slope and intercept into locals and return no slope/intercept when
either isn't finite. The existing null checks downstream then correctly treat the
regression as "no line": no line and no equation. The intercept-locked branch
gets the same guard for its own degenerate case (all points at the origin).

Testing

• New unit tests for leastSquaresLinearRegression covering normal data, fewer
  than two points, identical points, vertically aligned points, horizontally
  aligned points, and both intercept-locked cases.
• New LSRLAdornmentModel.computeValues tests confirming that a degenerate fit
  yields undefined line values and an invalid stored line.
• Verified in the browser: before the fix, two identical points reproduce
  x = (not a number); after, no line and no equation. Non-degenerate data still
  renders its line and equation correctly.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.69%. Comparing base (ed1154a) to head (739dee8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2674      +/-   ##
==========================================
- Coverage   87.69%   87.69%   -0.01%     
==========================================
  Files         810      810              
  Lines       46933    46938       +5     
  Branches    11562    12013     +451     
==========================================
+ Hits        41160    41162       +2     
- Misses       5759     5760       +1     
- Partials       14       16       +2     

Flag	Coverage Δ
cypress	69.50% <100.00%> (-0.01%)	⬇️
jest	62.66% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[cypress]

codap-v3    Run #12079

Run Properties:   Passed #12079  •  739dee8cab: null

Project	codap-v3
Branch Review	CODAP-1461-lsrl-degenerate-equation
Run status	Passed #12079
Run duration	09m 05s
Commit	739dee8cab: null
Committer	null
View all properties for this run ↗︎

---

Test results
Failures	0
Flaky	0
Pending	82
Skipped	0
Passing	382
View all changes introduced in this branch ↗︎

[Copilot]

Pull request overview

Fixes a degenerate least-squares regression case in the v3 graph (scatter plot) so that when no valid LSRL can be fit (e.g. identical or vertically aligned points), the LSRL adornment no longer shows a bogus equation like x = (not a number).

Changes:

• Updated leastSquaresLinearRegression() to avoid returning NaN slope/intercept by leaving them undefined when the computed values are not finite.
• Added unit tests for leastSquaresLinearRegression() covering normal, insufficient, degenerate, and intercept-locked cases.
• Added LSRLAdornmentModel.computeValues() tests to verify degenerate inputs yield undefined line values and an invalid stored line.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
v3/src/components/graph/utilities/graph-utils.ts	Guards regression results so degenerate fits don’t propagate NaN slope/intercept into rendering.
v3/src/components/graph/utilities/graph-utils.test.ts	Adds regression unit tests for normal/degenerate/intercept-locked scenarios.
v3/src/components/graph/adornments/lsrl/lsrl-adornment-model.test.ts	Adds tests ensuring degenerate LSRL results store as invalid and don’t produce equation/line values.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.