Decompose plot() (CC 33) and validate_input() (CC 31)

[tschm]

Subcategory: Code complexity
Score: 5 → 10

Average complexity is healthy — A (3.26) across 53 blocks, and all 10 modules rank A on the maintainability index. Two outliers drag the score down, both ranked E (radon's second-worst grade):

• src/pycharting/api/interface.py:75 plot — E (33), 13 parameters, ~37 branch points across a ~240-line body
• src/pycharting/data/ingestion.py:27 validate_input — E (31), 8 parameters

Both are wide optional-kwarg normalizers: a long sequential run of if arg is None / isinstance coercion branches. Suggested decomposition — extract per-concern helpers (array coercion, OHLC fallback derivation, overlay validation, subplot-spec validation, trade validation), or accept a params dataclass / Pydantic model instead of 13 positional-or-keyword arguments.

Also worth trimming while in the area (rank B, not currently failing):

• src/pycharting/core/lifecycle.py:99 _monitor_connection — B (9)
• src/pycharting/core/lifecycle.py:190 stop_server — B (9)
• src/pycharting/data/ingestion.py:328 DataManager.get_chunk — B (10)

done when

uvx radon cc src -a -s shows no block ranked worse than C (CC ≤ 10), with the 100% coverage gate still green.

Evidence

src/pycharting/api/interface.py
    F 75:0 plot - E (33)
src/pycharting/data/ingestion.py
    F 27:0 validate_input - E (31)
Average complexity: A (3.2641509433962264)

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Surfaced by the Rhiza quality gates during the template bump to jebel-quant/rhiza@v1.2.2, which enabled the stricter ruff rule families (ANN, A, BLE, ARG) and TYPECHECKER=both. Gate status at time of filing: make fmt FAIL, make typecheck FAIL, make docs-coverage FAIL, test-layout FAIL; make deptry, make security, make test PASS (154 tests, 100% coverage).