diff --git a/.coverage b/.coverage
deleted file mode 100644
index ec973e7..0000000
Binary files a/.coverage and /dev/null differ
diff --git a/.gitignore b/.gitignore
index 06af2e7..7b8b367 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,8 @@ build/
 .pytest_cache/
 .ruff_cache/
 .mypy_cache/
+.coverage
+htmlcov/
 
 # OS
 .DS_Store
diff --git a/docs/QA-2026-04-11_1132.md b/docs/QA-2026-04-11_1132.md
new file mode 100644
index 0000000..3f8c0f1
--- /dev/null
+++ b/docs/QA-2026-04-11_1132.md
@@ -0,0 +1,235 @@
+# Quality Assessment -- 2026-04-11
+
+> **Score: 83.7 / 100 — Grade: B+**
+> Codebase remains at B+ with no material change (-0.0 points) from the previous assessment; all eight dimension scores held steady — lint is still perfect, architecture is fully exposed, and the remaining paths to A-grade (coverage ≥80%, type errors <10, fewer files >300 lines) are the same priority items identified last cycle.
+
+| Dimension | Score |
+|-----------|-------|
+| Test Health | 8.0/10 |
+| Type Safety | 6.8/10 |
+| Lint Hygiene | 10.0/10 |
+| Architecture | 10.0/10 |
+| Documentation | 9.0/10 |
+| Complexity | 6.5/10 |
+| Security | 8.5/10 |
+| Maintainability | 8.5/10 |
+
+**Version:** 0.1.0
+**Assessed by:** Claude Sonnet 4.6
+**Previous assessment:** QA-2026-04-11_1054.md
+
+---
+
+## Inventory
+
+| Metric | Value |
+|--------|-------|
+| Source files | 72 |
+| Source lines | 10,575 |
+| Test files | 30 |
+| Test lines | 2,736 |
+| Test:source ratio | 0.259 |
+| Direct dependencies | 10 core + 1 optional + 4 dev |
+
+---
+
+## Raw Metrics
+
+### Test Suite
+
+```
+src/impakt/web/components/transforms.py             18     11    39%   102-164
+src/impakt/web/state.py                            152     11    93%   86, 96, 102, 112, 150, 201, 217, 223, 273-275
+------------------------------------------------------------------------------
+TOTAL                                             3763   1150    69%
+
+20 files skipped due to complete coverage.
+Required test coverage of 60% reached. Total coverage: 69.44%
+240 passed, 7 warnings in 16.77s
+```
+
+- Tests collected: 240
+- Tests passed: 240
+- Tests failed: 0
+- Test duration: 16.77s
+- Coverage: 69.44% (60% floor enforced via pytest-cov)
+
+### Type Safety (mypy --ignore-missing-imports)
+
+```
+src/impakt/web/callbacks/channel_callbacks.py:103: error: Incompatible return value type (got "NoReturn", expected "str | None")  [return-value]
+src/impakt/web/callbacks/export_callbacks.py:85: error: Module "dash.dcc" does not explicitly export attribute "send_data_frame"  [attr-defined]
+src/impakt/web/callbacks/export_callbacks.py:85: error: Call to untyped function "send_data_frame" in typed context  [no-untyped-call]
+src/impakt/web/callbacks/cursor_callbacks.py:25: error: Call to untyped function "clientside_callback" in typed context  [no-untyped-call]
+Found 34 errors in 16 files (checked 72 source files)
+```
+
+- Total errors: 34 (unchanged from previous)
+- Files with errors: 16 / 72 (78% clean)
+- Top error categories:
+  - `[attr-defined]` 9 — attribute access on loosely typed objects (Dash stubs)
+  - `[no-any-return]` 6 — returning `Any` from typed functions
+  - `[return-value]` 4 — incompatible return type
+  - `[import-untyped]` 3 — third-party stubs missing
+  - `[assignment]` 3 — incompatible assignment
+  - `[var-annotated]` 2 — need type annotation
+  - `[valid-type]` 2 — invalid type expression
+  - `[no-untyped-call]` 2 — call to untyped function
+  - `[unused-ignore]` 1
+  - `[no-untyped-def]` 1
+  - `[comparison-overlap]` 1
+
+### Lint (ruff)
+
+```
+All checks passed!
+```
+
+- Total violations: 0
+- Auto-fixable: N/A
+- Top violation rules: none
+
+### Complexity
+
+- File size: min=1 / median=132 / mean=147 / max=692
+- Files >300 lines: 8 / 72
+- High-complexity files (branch density >15):
+
+```
+  80  src/impakt/io/mme.py (692 lines)                              -- ISO 13499 parser, justified
+  44  src/impakt/web/components/criteria.py (342 lines)             -- UI assembly with protocol logic
+  32  src/impakt/web/callbacks/plot_callbacks.py (317 lines)        -- transform pipeline orchestration
+  30  src/impakt/channel/model.py (456 lines)                       -- core data model, multiple classes
+  27  src/impakt/web/state.py (275 lines)                           -- app state with multi-test support
+  27  src/impakt/protocol/euro_ncap.py (237 lines)                  -- sliding-scale scoring tables
+  26  src/impakt/plot/engine.py (316 lines)                         -- Plotly rendering with corridors
+  21  src/impakt/web/callbacks/channel_callbacks.py (236 lines)     -- selection/filter callbacks
+  21  src/impakt/protocol/iihs.py (179 lines)                       -- G/A/M/P rating logic
+  20  src/impakt/web/components/channel_grid.py (418 lines)         -- DataTable assembly
+  19  src/impakt/script/cli.py (137 lines)                          -- CLI arg parsing
+```
+
+### Documentation
+
+- Docstring coverage: 420 / 457 definitions (91.9%)
+- Modules with `__all__`: 11 / 11 (all modules)
+  - channel: YES
+  - criteria: YES
+  - io: YES
+  - plot: YES
+  - plugin: YES
+  - protocol: YES
+  - report: YES
+  - script: YES
+  - template: YES
+  - transform: YES
+  - web: YES
+- README: 1,266 lines with Mermaid diagrams
+- Architectural diagrams: yes
+
+### Security
+
+```
+eval/exec (all hits):
+  src/impakt/transform/math_expr.py:149: eval(self.expression, {"__builtins__": {}}, namespace)  # noqa: S307
+
+subprocess/os.system hits:
+  src/impakt/transform/math_expr.py:68:        "subprocess",   ← string in forbidden-token blocklist, NOT a call
+```
+
+- eval/exec (sandboxed): 1 — `math_expr.py:149`, restricted builtins `{}` + 16-token blocklist; noqa: S307
+- eval/exec (unsandboxed): 0 (grep excluding # noqa returns 0)
+- subprocess: 1 confirmed false positive — line 68 is a blocklist string entry, not an invocation
+- Hardcoded secrets: 0
+- Bare except: 0
+
+### Maintainability
+
+```
+TODO: 0
+FIXME: 0
+HACK: 0
+logging calls: 50
+try/except blocks: 53
+bare excepts: 0
+internal imports: 198
+```
+
+- TODO: 0
+- FIXME: 0
+- HACK: 0
+- Logging calls: 50
+- try/except blocks: 53
+- Bare excepts: 0
+- Internal imports (coupling): 198
+
+---
+
+## Scorecard
+
+| # | Dimension | Weight | Score | Weighted | Justification |
+|---|-----------|--------|-------|----------|---------------|
+| 1 | Test Health | 20% | 8.0/10 | 1.60 | 240/240 pass; test:source ratio 0.259 (within 0.2–0.5 band); integration tests with real datasets present; 69.44% coverage measured (below 80% ceiling for a 9+). Matches rubric row "100% pass, ratio 0.2–0.5, integration tests present" = 8. |
+| 2 | Type Safety | 15% | 6.8/10 | 1.02 | mypy (ignore-missing-imports), 34 errors in 16 files — unchanged. Linear interpolation between 6 (<50 errors) and 8 (<10 errors): 6 + (50−34)/(50−10) × 2 = 6.8. |
+| 3 | Lint Hygiene | 10% | 10.0/10 | 1.00 | `ruff check src/` reports 0 violations — perfect score maintained. |
+| 4 | Architecture | 15% | 10.0/10 | 1.50 | Clear 4-layer design (io → transform/channel → protocol → web/plot). Plugin system present. All 11 public modules export `__all__`. No layer violations confirmed (io/transform/protocol/channel contain no imports from web or plot). |
+| 5 | Documentation | 10% | 9.0/10 | 0.90 | 91.9% docstring coverage exceeds 90% threshold. Comprehensive README with Mermaid diagrams. No generated API reference (Sphinx/mkdocs), so not a full 10. |
+| 6 | Complexity | 10% | 6.5/10 | 0.65 | Median 132 (<150, excellent). 8 files >300 lines (unchanged). Interpolated between score 8 (≤3 files) and 6 (≤10 files): 6 + (10−8)/(10−3) × 2 = 6.57 → 6.5. |
+| 7 | Security | 10% | 8.5/10 | 0.85 | Single eval sandboxed with `{"__builtins__": {}}` + 16-item token blocklist; noqa annotation in place. Subprocess hit confirmed false positive. No secrets, no bare excepts. Interpolated between 9 (fully sandboxed) and 7 (partially sandboxed) for conservative assessment. |
+| 8 | Maintainability | 10% | 8.5/10 | 0.85 | Zero debt markers. Zero bare excepts. 50 logging calls. Modern tooling (uv, hatchling, ruff, mypy). Interpolated between 10 (0 markers + all criteria) and 8 (<5 markers) at 8.5 for consistency with prior assessments; high internal coupling (198) noted. |
+
+### Composite Score: **83.7 / 100**
+### Grade: **B+**
+
+Calculation: (8.0×0.20 + 6.8×0.15 + 10.0×0.10 + 10.0×0.15 + 9.0×0.10 + 6.5×0.10 + 8.5×0.10 + 8.5×0.10) × 10
+= (1.60 + 1.02 + 1.00 + 1.50 + 0.90 + 0.65 + 0.85 + 0.85) × 10
+= 8.37 × 10 = **83.7**
+
+---
+
+## Delta from Previous Assessment
+
+| Dimension | Previous | Current | Change |
+|-----------|----------|---------|--------|
+| Test Health | 8.0 | 8.0 | 0.0 |
+| Type Safety | 6.8 | 6.8 | 0.0 |
+| Lint Hygiene | 10.0 | 10.0 | 0.0 |
+| Architecture | 10.0 | 10.0 | 0.0 |
+| Documentation | 9.0 | 9.0 | 0.0 |
+| Complexity | 6.5 | 6.5 | 0.0 |
+| Security | 8.5 | 8.5 | 0.0 |
+| Maintainability | 8.5 | 8.5 | 0.0 |
+| **Composite** | **83.7** | **83.7** | **0.0** |
+
+---
+
+## Top Improvements Since Last Assessment
+
+1. **No regression** — All dimensions held steady; the 8-file >300-lines count is unchanged, coverage held at ~69.4%, and 34 mypy errors remain.
+2. **Minor source growth** — +12 lines (10,563 → 10,575) with no new files or test changes; composition is stable.
+3. **Coverage delta negligible** — 69.51% → 69.44% (−0.07 pp); within measurement noise.
+
+---
+
+## Recommended Actions (Priority Order)
+
+| # | Action | Effort | Impact | Dimensions Affected |
+|---|--------|--------|--------|---------------------|
+| 1 | Increase test coverage from 69.44% to ≥80%: add unit tests for uncovered branches in `web/components/transforms.py` (39% covered), `plot/engine.py`, and `io/mme.py` parser edge-cases | 2–4 hr | Test Health +1.0 → 9.0 (+0.20 composite) | Test Health |
+| 2 | Resolve 9 `[attr-defined]` + 6 `[no-any-return]` + 4 `[return-value]` mypy errors in the web layer to reach <10 total errors; consider adding Dash type stubs or local overrides | 1–2 hr | Type Safety +1.2 → 8.0 (+0.18 composite) | Type Safety |
+| 3 | Decompose `channel_grid.py` (418 lines) and `channel/model.py` (456 lines) into focused sub-modules; extract sub-parsers from `mme.py` if it grows beyond 800 lines | 3–5 hr | Complexity +0.5 → 7.0 (+0.05 composite) | Complexity |
+| 4 | Set up mkdocs-material + mkdocstrings to auto-generate API reference from existing docstrings (91.9% coverage makes this low-friction) | 1–2 hr | Documentation +1.0 → 10.0 (+0.10 composite) | Documentation |
+| 5 | Replace `eval`-based math expression evaluator in `math_expr.py` with an AST-based parser (`ast.parse` + safe node visitor) to eliminate last eval usage entirely | 2–3 hr | Security +0.5 → 9.0 (+0.05 composite) | Security |
+
+**Projected composite after actions 1–4: ~87.0 (B+, approaching A threshold); after all 5: ~87.5 (B+)**
+
+---
+
+## Notes
+
+- **All scores unchanged from QA-2026-04-11_1054.md.** The codebase added 12 source lines with no structural changes. This is a stability confirmation assessment, not a milestone improvement run.
+- **Architecture qualitatively verified.** All 11 `__init__.py` files confirmed present with `__all__`. Grep for web/plot imports in io/transform/channel/protocol layers returned zero hits — no layer violations exist.
+- **Security subprocess false positive persists.** `math_expr.py:68` contains `"subprocess"` as a string entry in its forbidden-token blocklist, not an invocation. The grep count of 1 is expected and benign.
+- **eval sandboxing confirmed.** The single `eval` call at `math_expr.py:149` uses `{"__builtins__": {}}` (removes all builtins) plus the 16-token text blocklist scanned before evaluation. The `# noqa: S307` suppresses the ruff/bandit rule correctly.
+- **Coverage measurement note.** The `--co` (collect-only) run reports 30.67% because 11 files are skipped during collection; the full run gives 69.44%. Use only the full-run number for scoring.
+- **Next A-grade path.** Reaching 90+ requires: coverage ≥80% (+0.20), type errors <10 (+0.18), and generated API docs (+0.10) = minimum +0.48 composite → ~87–88. Getting to 90 additionally requires complexity improvement (more decomposition) or maintainability nudge.
diff --git a/pyproject.toml b/pyproject.toml
index c1cba9e..237a429 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -36,7 +36,7 @@ dependencies = [
     "weasyprint>=60.0",
     "pydantic>=2.0",
     "plotly-resampler>=0.11.0",
-    "pytz>=2026.1.post1",
+    "pytz>=2024.1",  # required by plotly-resampler (not declared in its deps)
 ]
 
 [project.optional-dependencies]
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diff --git a/src/impakt/plot/engine.py b/src/impakt/plot/engine.py
index 03736ce..6780eb3 100644
--- a/src/impakt/plot/engine.py
+++ b/src/impakt/plot/engine.py
@@ -52,9 +52,7 @@ class PlotEngine:
     aggregation on zoom/pan events.
     """
 
-    def render(
-        self, spec: PlotSpec, *, resample: bool = False
-    ) -> go.Figure | FigureResampler:
+    def render(self, spec: PlotSpec, *, resample: bool = False) -> go.Figure | FigureResampler:
         """Render a PlotSpec into an interactive Plotly figure.
 
         Args:
@@ -97,8 +95,19 @@ class PlotEngine:
         for corridor in spec.corridors:
             self._add_corridor(fig, corridor, resample=resample)
 
+        # Determine render order: non-focus traces first, focus trace last (on top)
+        focus_idx = spec.focus_index
+        if focus_idx is not None and focus_idx < 0:
+            focus_idx = len(spec.channels) + focus_idx  # resolve negative index
+
+        render_order = list(range(len(spec.channels)))
+        if focus_idx is not None and 0 <= focus_idx < len(spec.channels):
+            render_order.remove(focus_idx)
+            render_order.append(focus_idx)
+
         # Add channel traces
-        for i, ch_ref in enumerate(spec.channels):
+        for i in render_order:
+            ch_ref = spec.channels[i]
             ch = ch_ref.channel
             if ch is None:
                 continue
@@ -111,12 +120,18 @@ class PlotEngine:
             color = style.color or DEFAULT_COLORS[i % len(DEFAULT_COLORS)]
             label = style.label or ch_ref.label
 
+            # Focus trace gets enhanced styling
+            is_focus = i == focus_idx
+            line_width = 2.5 if is_focus else style.line_width
+            if is_focus and not style.color:
+                color = "#ffc107"  # amber — focus highlight
+
             trace_kwargs: dict[str, Any] = {
                 "mode": "lines",
                 "name": label,
                 "line": dict(
                     color=color,
-                    width=style.line_width,
+                    width=line_width,
                     dash=style.line_dash,
                 ),
                 "opacity": style.opacity,
@@ -128,8 +143,6 @@ class PlotEngine:
                 )
 
             if resample:
-                # FigureResampler: pass full-resolution data via hf_x/hf_y
-                # and use Scattergl for WebGL-accelerated rendering
                 fig.add_trace(
                     go.Scattergl(**trace_kwargs),
                     hf_x=ch.time,
diff --git a/src/impakt/plot/spec.py b/src/impakt/plot/spec.py
index abb7039..031a17f 100644
--- a/src/impakt/plot/spec.py
+++ b/src/impakt/plot/spec.py
@@ -173,3 +173,6 @@ class PlotSpec:
     compact: bool = False
     hovermode: str | bool = "x unified"
     margin: dict[str, int] | None = None
+    # Focus: index into channels list to render last (on top) with
+    # thicker line. None means no focus. -1 means last channel.
+    focus_index: int | None = None
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diff --git a/src/impakt/transform/__pycache__/resultant.cpython-312.pyc b/src/impakt/transform/__pycache__/resultant.cpython-312.pyc
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diff --git a/src/impakt/web/callbacks/plot_callbacks.py b/src/impakt/web/callbacks/plot_callbacks.py
index a2a95f6..ad4882f 100644
--- a/src/impakt/web/callbacks/plot_callbacks.py
+++ b/src/impakt/web/callbacks/plot_callbacks.py
@@ -38,10 +38,6 @@ logger = logging.getLogger(__name__)
 # Module-level engine instance (stateless, safe to reuse)
 _engine = PlotEngine()
 
-# Server-side storage for the current FigureResampler instance.
-# Single-user desktop app — no multi-tenant isolation needed.
-_current_resampler: FigureResampler | None = None
-
 
 def _build_transform_chain(
     cfc_value: str,
@@ -160,19 +156,16 @@ def _build_plot_spec(
     cursor_x1: float | None,
     cursor_x2: float | None,
     corridors: list[dict[str, Any]] | None = None,
-    focus_key: str | None = None,
-    focus_channel: tuple[str, Channel] | None = None,
+    focus_index: int | None = None,
 ) -> PlotSpec:
     """Build a PlotSpec from resolved channels and UI state.
 
     Channels are already transformed — they are wrapped in ChannelRef
-    objects with no additional transform chain (transforms were applied
-    during resolution).
+    objects with no additional transform chain.
 
-    If ``focus_channel`` is provided, it is rendered last (on top) with
-    a thicker line so it stands out from pinned channels.
+    ``focus_index`` is the index into ``channels`` of the focused channel.
+    PlotEngine renders this trace last (on top) with enhanced styling.
     """
-    # Build ChannelRef objects — pinned channels first (thinner)
     refs: list[ChannelRef] = []
     for i, (label, ch) in enumerate(channels):
         color = DEFAULT_COLORS[i % len(DEFAULT_COLORS)]
@@ -183,20 +176,6 @@ def _build_plot_spec(
             )
         )
 
-    # Add focus channel last so it renders on top
-    if focus_channel is not None:
-        f_label, f_ch = focus_channel
-        refs.append(
-            ChannelRef(
-                channel=f_ch,
-                style=PlotStyle(
-                    label=f_label,
-                    color="#ffc107",  # amber — matches focus highlight in grid
-                    line_width=2.5,
-                ),
-            )
-        )
-
     # Build Corridor objects from raw dicts
     corridor_objs: list[Corridor] = []
     if corridors:
@@ -226,13 +205,13 @@ def _build_plot_spec(
         corridors=corridor_objs,
         x_cursors=x_cursors,
         y_label=y_label,
-        compact=True,  # Web UI always uses compact mode
+        compact=True,
+        focus_index=focus_index,
     )
 
 
 def register_plot_callbacks(app: dash.Dash, app_state: AppState) -> None:
     """Register all plot-related callbacks."""
-    global _current_resampler
 
     @app.callback(
         Output({"type": "plot-graph", "index": 0}, "figure"),
@@ -280,31 +259,45 @@ def register_plot_callbacks(app: dash.Dash, app_state: AppState) -> None:
             show_resultant,
         )
 
-        # Resolve focus channel (if not already pinned)
-        focus_channel: tuple[str, Channel] | None = None
-        if focus_key and focus_key not in (selected_keys or []):
-            focus_resolved = _resolve_channels(
-                [focus_key],
-                app_state,
-                cfc_value,
-                y_align,
-                x_align_method or "none",
-                x_align_value,
-                False,  # no resultant for single focus
-            )
-            if focus_resolved:
-                focus_channel = focus_resolved[0]
+        # Resolve focus: if focus_key is one of the selected channels,
+        # find its index. If it's not selected, add it to the channels list
+        # and set focus_index to the new last position.
+        focus_idx: int | None = None
+        if focus_key:
+            # Check if focus is already in the selected channels
+            for i, (_, ch) in enumerate(channels):
+                ch_key = f"{ch.source_test_id}::{ch.name}" if ch.source_test_id else ch.name
+                if ch_key == focus_key:
+                    focus_idx = i
+                    break
+
+            # If not in selected, resolve and append
+            if focus_idx is None:
+                focus_resolved = _resolve_channels(
+                    [focus_key],
+                    app_state,
+                    cfc_value,
+                    y_align,
+                    x_align_method or "none",
+                    x_align_value,
+                    False,
+                )
+                if focus_resolved:
+                    channels.append(focus_resolved[0])
+                    focus_idx = len(channels) - 1
 
         spec = _build_plot_spec(
-            channels, cursor_x1, cursor_x2, corridors_data,
-            focus_key=focus_key,
-            focus_channel=focus_channel,
+            channels,
+            cursor_x1,
+            cursor_x2,
+            corridors_data,
+            focus_index=focus_idx,
         )
         fig = _engine.render(spec, resample=True)
 
-        # Store the resampler for relayoutData callbacks
+        # Store the resampler in AppState for relayoutData callbacks
         if isinstance(fig, FigureResampler):
-            _current_resampler = fig
+            app_state.current_resampler = fig
 
         return fig
 
@@ -319,6 +312,6 @@ def register_plot_callbacks(app: dash.Dash, app_state: AppState) -> None:
         FigureResampler intercepts relayoutData events and returns a
         Patch that updates only the visible trace data — no full re-render.
         """
-        if _current_resampler is None or relayout_data is None:
+        if app_state.current_resampler is None or relayout_data is None:
             return no_update
-        return _current_resampler.construct_update_data_patch(relayout_data)
+        return app_state.current_resampler.construct_update_data_patch(relayout_data)
diff --git a/src/impakt/web/state.py b/src/impakt/web/state.py
index 8c81ec0..552703f 100644
--- a/src/impakt/web/state.py
+++ b/src/impakt/web/state.py
@@ -37,6 +37,9 @@ class AppState:
         self._active_template: TemplateSpec | None = None
         # Per-channel transform overrides: {channel_key: {"cfc": "600"}}
         self.channel_overrides: dict[str, dict[str, Any]] = {}
+        # Current FigureResampler instance for zoom/pan resampling.
+        # Stored here (not as a module global) so it's per-AppState.
+        self.current_resampler: Any = None
         # Active corridors
         self.corridors: list[dict[str, Any]] = []
 
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diff --git a/tests/test_transform/__pycache__/test_align.cpython-312-pytest-9.0.3.pyc b/tests/test_transform/__pycache__/test_align.cpython-312-pytest-9.0.3.pyc
deleted file mode 100644
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diff --git a/tests/test_transform/__pycache__/test_cfc.cpython-312-pytest-9.0.3.pyc b/tests/test_transform/__pycache__/test_cfc.cpython-312-pytest-9.0.3.pyc
deleted file mode 100644
index ae1423e..0000000
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diff --git a/uv.lock b/uv.lock
index f20c5ef..1562517 100644
--- a/uv.lock
+++ b/uv.lock
@@ -567,7 +567,7 @@ requires-dist = [
     { name = "plotly", specifier = ">=5.18" },
     { name = "plotly-resampler", specifier = ">=0.11.0" },
     { name = "pydantic", specifier = ">=2.0" },
-    { name = "pytz", specifier = ">=2026.1.post1" },
+    { name = "pytz", specifier = ">=2024.1" },
     { name = "pyyaml", specifier = ">=6.0" },
     { name = "scipy", specifier = ">=1.10" },
     { name = "weasyprint", specifier = ">=60.0" },