feat: Trends drill-down — click directorate to see drug-level trends (Task E.4)

IMPLEMENTATION_PLAN.md

@@ -294,22 +294,18 @@ Comprehensive review and improvement of all Plotly charts in the Dash dashboard.
 - **Checkpoint**: Trends view shows directorate-level line chart. Metric toggle switches y-axis. Lines show one per directorate. PASSED.
 
 ### E.4 Add drug drill-down within Trends view
-- [ ] Add `selected_trends_directorate` key (default `None`) to `app-state` initial data in `app.py`
-- [ ] Add directorate selection callback in `dash_app/callbacks/trends.py`:
-  - Clicking a line/trace on the overview chart sets `selected_trends_directorate` in app-state
-  - Use `clickData` from `trends-overview-chart` as Input
-  - Extract directorate name from the clicked trace's `name` attribute
-  - Update `app-state` with `selected_trends_directorate`
-- [ ] Add landing/detail toggle callback:
-  - Input: `app-state` → show/hide `trends-landing` vs `trends-detail`
-  - When `selected_trends_directorate` is set: hide landing, show detail with title "[Directorate] — Drug Trends"
-- [ ] Add detail chart callback:
-  - Input: `app-state` + `trends-view-metric-toggle` → Output `trends-detail-chart`
+- [x] Add `selected_trends_directorate` key (default `None`) to `app-state` initial data in `app.py` (already done in E.2)
+- [x] Add `customdata=[name]*len(periods)` to each trace in `create_trend_figure()` so directorate name is accessible from clickData
+- [x] Add `Input("trends-overview-chart", "clickData")` and `Input("trends-back-btn", "n_clicks")` to `update_app_state()` in `filters.py`:
+  - Clicking a trace point extracts directorate name from `clickData["points"][0]["customdata"]`
+  - Back button clears `selected_trends_directorate` to None
+  - Chart type change also clears `selected_trends_directorate`
+- [x] Landing/detail toggle callback already exists in `trends.py` (`toggle_trends_subviews`) — handles show/hide based on `selected_trends_directorate`
+- [x] Add `render_trends_detail()` callback in `trends.py`:
+  - Input: `app-state` + `trends-detail-metric-toggle` → Output `trends-detail-chart`
   - Calls `get_trend_data(directory=selected, metric=..., group_by="drug")` → `create_trend_figure()`
-  - Only fires when `selected_trends_directorate` is not None
-- [ ] Add back button callback:
-  - Clicking `trends-back-btn` clears `selected_trends_directorate` in app-state → returns to landing
-- **Checkpoint**: Click a directorate line → drill into drug-level trends. Back button returns to overview. `python run_dash.py` starts cleanly.
+  - Guards: only fires when `active_view == "trends"` and `selected_trends_directorate` is not None
+- **Checkpoint**: Click a directorate line → drill into drug-level trends. Back button returns to overview. `python run_dash.py` starts cleanly. PASSED.
 
 ### E.5 Fix chart height to fill viewport
 - [ ] In `create_trend_figure()` in `plotly_generator.py`: remove explicit `height=500`, let `autosize=True` (from `_base_layout()`) handle it
@@ -357,8 +353,8 @@ Comprehensive review and improvement of all Plotly charts in the Dash dashboard.
 - [x] Trends tab removed from Patient Pathways (9 tabs remain)
 - [x] 3rd sidebar item "Trends" visible and functional
 - [x] Trends landing page shows directorate-level line chart with metric toggle
-- [ ] Clicking a directorate drills into drug-level trends
-- [ ] Back button returns to directorate overview
+- [x] Clicking a directorate drills into drug-level trends
+- [x] Back button returns to directorate overview
 - [ ] Charts fill available viewport height (no fixed 500px cutoff)
 - [x] "Cost" renamed to "Cost per Patient" in metric toggles
 - [ ] `python run_dash.py` starts cleanly

dash_app/callbacks/filters.py

@@ -78,12 +78,15 @@ def register_filter_callbacks(app):
         Input("nav-trends", "n_clicks"),
         Input({"type": "tc-selector", "index": ALL}, "n_clicks"),
         Input("tc-back-btn", "n_clicks"),
+        Input("trends-overview-chart", "clickData"),
+        Input("trends-back-btn", "n_clicks"),
         State("app-state", "data"),
     )
     def update_app_state(
         _dir_clicks, _ind_clicks, initiated, last_seen, selected_drugs,
         selected_trusts, _nav_pp_clicks, _nav_tc_clicks, _nav_trends_clicks,
-        _tc_selector_clicks, _tc_back_clicks, current_state
+        _tc_selector_clicks, _tc_back_clicks, trends_click_data,
+        _trends_back_clicks, current_state
     ):
         """Update app-state when any filter, nav, or TC selector changes."""
         if not current_state:
@@ -134,6 +137,21 @@ def register_filter_callbacks(app):
         if chart_type != prev_chart_type and selected_comparison_directorate is not None:
             selected_comparison_directorate = None
 
+        # Trends directorate drill-down
+        selected_trends_directorate = current_state.get("selected_trends_directorate")
+
+        if triggered_id == "trends-overview-chart" and trends_click_data:
+            points = trends_click_data.get("points", [])
+            if points:
+                selected_trends_directorate = points[0].get("customdata")
+
+        if triggered_id == "trends-back-btn":
+            selected_trends_directorate = None
+
+        # If chart type changed while trends directorate is selected, return to landing
+        if chart_type != prev_chart_type and selected_trends_directorate is not None:
+            selected_trends_directorate = None
+
         # Compute date_filter_id from dropdown values
         date_filter_id = f"{initiated}_{last_seen}"
 
@@ -148,6 +166,7 @@ def register_filter_callbacks(app):
             "selected_trusts": selected_trusts or [],
             "active_view": active_view,
             "selected_comparison_directorate": selected_comparison_directorate,
+            "selected_trends_directorate": selected_trends_directorate,
         }
 
         # Toggle pill CSS classes

dash_app/callbacks/trends.py

@@ -92,3 +92,49 @@ def register_trends_callbacks(app):
         title = f"Directorate Trends \u2014 {metric_labels.get(metric, 'Patients')}"
 
         return create_trend_figure(data, title, metric)
+
+    # --- Drug detail chart (drill-down) ---
+    @app.callback(
+        Output("trends-detail-chart", "figure"),
+        Input("app-state", "data"),
+        Input("trends-detail-metric-toggle", "value"),
+        prevent_initial_call=True,
+    )
+    def render_trends_detail(app_state, metric):
+        """Render drug-level trends for the selected directorate."""
+        if not app_state:
+            return no_update
+
+        active_view = app_state.get("active_view", "")
+        if active_view != "trends":
+            return no_update
+
+        selected = app_state.get("selected_trends_directorate")
+        if not selected:
+            return no_update
+
+        metric = metric or "patients"
+
+        from dash_app.data.queries import get_trend_data
+        from visualization.plotly_generator import create_trend_figure
+
+        try:
+            data = get_trend_data(
+                metric=metric,
+                directory=selected,
+                group_by="drug",
+            )
+        except Exception:
+            return _trends_empty("Failed to load trend data.")
+
+        if not data:
+            return _trends_empty("No trend data for this directorate.")
+
+        metric_labels = {
+            "patients": "Patients",
+            "total_cost": "Cost per Patient",
+            "cost_pp_pa": "Cost per Patient p.a.",
+        }
+        title = f"{selected} \u2014 {metric_labels.get(metric, 'Patients')}"
+
+        return create_trend_figure(data, title, metric)

src/visualization/plotly_generator.py

@@ -2343,6 +2343,7 @@ def create_trend_figure(
             y=s["values"],
             mode="lines+markers",
             name=name,
+            customdata=[name] * len(s["periods"]),
             line=dict(color=colour, width=2),
             marker=dict(color=colour, size=6),
             hovertemplate=(