feat: duration vs cost scatter plot tab (Task C.3)

IMPLEMENTATION_PLAN.md

@@ -160,16 +160,18 @@ Comprehensive review and improvement of all Plotly charts in the Dash dashboard.
 - **Checkpoint**: Depth tab shows patient distribution by treatment line count
 
 ### C.3 Duration vs Cost scatter plot
-- [ ] Create `get_duration_cost_scatter()` in `src/data_processing/pathway_queries.py`:
-  - Query level 3 nodes for drug-level data
+- [x] Create `get_duration_cost_scatter()` in `src/data_processing/pathway_queries.py`:
+  - Query level 3 nodes for drug-level data with avg_days and cost_pp_pa
+  - Aggregates across trusts using weighted averages
   - Return: `[{drug, directory, avg_days, cost_pp_pa, patients}, ...]`
-- [ ] Add thin wrapper in `dash_app/data/queries.py`
-- [ ] Create `create_duration_cost_scatter_figure(data, title)` in `src/visualization/plotly_generator.py`:
-  - Scatter: x=avg_days, y=cost_pp_pa, size=patients, color=directory
-  - Add quadrant lines at median values (4 quadrants: cheap/short, cheap/long, expensive/short, expensive/long)
+- [x] Add thin wrapper in `dash_app/data/queries.py`
+- [x] Create `create_duration_cost_scatter_figure(data, title)` in `src/visualization/plotly_generator.py`:
+  - Scatter: x=avg_days, y=cost_pp_pa, size=patients (global max), color=directory
+  - One trace per directory for legend grouping using DRUG_PALETTE
+  - Quadrant lines at median values with annotations
   - Hover shows drug name, directory, all values
-- [ ] Add "Scatter" tab to `TAB_DEFINITIONS` in `chart_card.py`
-- [ ] Add `_render_scatter()` helper and tab dispatch in `dash_app/callbacks/chart.py`
+- [x] Add "Scatter" tab to `TAB_DEFINITIONS` in `chart_card.py` (6 tabs: Icicle, Sankey, Heatmap, Funnel, Depth, Scatter)
+- [x] Add `_render_scatter()` helper and tab dispatch in `dash_app/callbacks/chart.py`
 - **Checkpoint**: Scatter tab shows drugs by duration vs cost with directorate coloring
 
 ### C.4 Drug switching network graph

dash_app/callbacks/chart.py

@@ -317,6 +317,31 @@ def _render_depth(app_state, title):
     return create_pathway_depth_figure(data, title)
 
 
+def _render_scatter(app_state, title):
+    """Build the duration vs cost scatter plot from current filter state."""
+    from dash_app.data.queries import get_duration_cost_scatter
+    from visualization.plotly_generator import create_duration_cost_scatter_figure
+
+    filter_id = (app_state or {}).get("date_filter_id", "all_6mo")
+    chart_type = (app_state or {}).get("chart_type", "directory")
+
+    selected_dirs = (app_state or {}).get("selected_directorates") or []
+    selected_trusts = (app_state or {}).get("selected_trusts") or []
+    directory = selected_dirs[0] if len(selected_dirs) == 1 else None
+    trust = selected_trusts[0] if len(selected_trusts) == 1 else None
+
+    try:
+        data = get_duration_cost_scatter(filter_id, chart_type, directory, trust)
+    except Exception:
+        log.exception("Failed to load duration/cost scatter data")
+        return _empty_figure("Failed to load scatter data.")
+
+    if not data:
+        return _empty_figure("No duration/cost data available.\nTry adjusting your filters.")
+
+    return create_duration_cost_scatter_figure(data, title)
+
+
 def register_chart_callbacks(app):
     """Register tab switching, pathway data loading, and chart rendering callbacks."""
 
@@ -463,6 +488,9 @@ def register_chart_callbacks(app):
         elif active_tab == "depth":
             fig = _render_depth(app_state, title)
 
+        elif active_tab == "scatter":
+            fig = _render_scatter(app_state, title)
+
         else:
             # Placeholder for charts not yet implemented
             tab_label = dict(TAB_DEFINITIONS).get(active_tab, active_tab)

dash_app/components/chart_card.py

@@ -10,6 +10,7 @@ TAB_DEFINITIONS = [
     ("heatmap", "Heatmap"),
     ("funnel", "Funnel"),
     ("depth", "Depth"),
+    ("scatter", "Scatter"),
 ]
 
 # Full set retained for Trust Comparison dashboard (Phase 10.8)

dash_app/data/queries.py

@@ -26,6 +26,7 @@ from data_processing.pathway_queries import (
     get_directorate_summary as _get_directorate_summary,
     get_retention_funnel as _get_retention_funnel,
     get_pathway_depth_distribution as _get_pathway_depth_distribution,
+    get_duration_cost_scatter as _get_duration_cost_scatter,
 )
 
 DB_PATH = Path(__file__).resolve().parents[2] / "data" / "pathways.db"
@@ -205,3 +206,13 @@ def get_pathway_depth_distribution(
 ) -> list[dict]:
     """Patients who stopped at each treatment line depth (exclusive counts)."""
     return _get_pathway_depth_distribution(DB_PATH, date_filter_id, chart_type, directory, trust)
+
+
+def get_duration_cost_scatter(
+    date_filter_id: str = "all_6mo",
+    chart_type: str = "directory",
+    directory: Optional[str] = None,
+    trust: Optional[str] = None,
+) -> list[dict]:
+    """Drug-level avg_days and cost_pp_pa for scatter plot."""
+    return _get_duration_cost_scatter(DB_PATH, date_filter_id, chart_type, directory, trust)

src/data_processing/pathway_queries.py

@@ -1209,6 +1209,82 @@ def get_pathway_depth_distribution(
         conn.close()
 
 
+def get_duration_cost_scatter(
+    db_path: Path,
+    date_filter_id: str,
+    chart_type: str,
+    directory: Optional[str] = None,
+    trust: Optional[str] = None,
+) -> list[dict]:
+    """Level 3 drug nodes with avg_days and cost_pp_pa for scatter plot.
+
+    Returns list of dicts: [{drug, directory, avg_days, cost_pp_pa, patients}]
+    Excludes nodes missing avg_days or cost_pp_pa. Aggregates across trusts
+    using weighted averages.
+    """
+    conn = sqlite3.connect(str(db_path))
+    conn.row_factory = sqlite3.Row
+    try:
+        where = ["date_filter_id = ?", "chart_type = ?", "level = 3",
+                 "avg_days IS NOT NULL", "cost_pp_pa IS NOT NULL"]
+        params: list = [date_filter_id, chart_type]
+
+        if directory:
+            where.append("directory = ?")
+            params.append(directory)
+        if trust:
+            where.append("trust_name = ?")
+            params.append(trust)
+
+        query = f"""
+            SELECT labels AS drug, directory,
+                   value AS patients, avg_days, cost_pp_pa
+            FROM pathway_nodes
+            WHERE {' AND '.join(where)}
+        """
+        rows = conn.execute(query, params).fetchall()
+
+        # Aggregate across trusts: weighted average of avg_days and cost_pp_pa
+        agg = {}
+        for r in rows:
+            key = (r["directory"] or "", r["drug"])
+            patients = r["patients"] or 0
+            days = _safe_float(r["avg_days"])
+            cost = _safe_float(r["cost_pp_pa"])
+            if patients == 0 or days == 0:
+                continue
+
+            if key not in agg:
+                agg[key] = {
+                    "drug": r["drug"],
+                    "directory": r["directory"] or "",
+                    "weighted_days": 0.0,
+                    "weighted_cost": 0.0,
+                    "total_patients": 0,
+                }
+            agg[key]["weighted_days"] += days * patients
+            agg[key]["weighted_cost"] += cost * patients
+            agg[key]["total_patients"] += patients
+
+        result = []
+        for v in agg.values():
+            tp = v["total_patients"]
+            if tp > 0:
+                result.append({
+                    "drug": v["drug"],
+                    "directory": v["directory"],
+                    "avg_days": round(v["weighted_days"] / tp, 1),
+                    "cost_pp_pa": round(v["weighted_cost"] / tp, 0),
+                    "patients": tp,
+                })
+
+        return result
+    except sqlite3.Error:
+        return []
+    finally:
+        conn.close()
+
+
 def get_directorate_summary(
     db_path: Path,
     date_filter_id: str,

src/visualization/plotly_generator.py

@@ -1891,3 +1891,109 @@ def create_pathway_depth_figure(
     fig.update_layout(**layout)
 
     return fig
+
+
+def create_duration_cost_scatter_figure(
+    data: list[dict],
+    title: str = "",
+) -> go.Figure:
+    """Create a Duration vs Cost scatter plot from drug-level data.
+
+    Each point represents a drug (within a directory). x=avg treatment days,
+    y=annualised cost per patient, size=patient count, color=directory.
+    Quadrant lines at median values divide into 4 regions.
+    """
+    if not data:
+        return go.Figure()
+
+    import statistics
+
+    display_title = f"Duration vs Cost — {title}" if title else "Duration vs Cost"
+
+    # Assign colors by directory
+    directories = sorted(set(d["directory"] for d in data))
+    dir_colors = {
+        d: DRUG_PALETTE[i % len(DRUG_PALETTE)]
+        for i, d in enumerate(directories)
+    }
+
+    # Global max patients for consistent sizing across directories
+    global_max_p = max((d["patients"] for d in data), default=1) or 1
+
+    # Build one trace per directory for legend grouping
+    fig = go.Figure()
+    for directory in directories:
+        subset = [d for d in data if d["directory"] == directory]
+        patients = [d["patients"] for d in subset]
+
+        # Scale marker size: min 8, max 40, relative to global max
+        sizes = [max(8, min(40, 8 + 32 * (p / global_max_p))) for p in patients]
+
+        fig.add_trace(go.Scatter(
+            x=[d["avg_days"] for d in subset],
+            y=[d["cost_pp_pa"] for d in subset],
+            mode="markers",
+            name=directory,
+            marker=dict(
+                size=sizes,
+                color=dir_colors[directory],
+                opacity=0.75,
+                line=dict(width=1, color="white"),
+            ),
+            text=[d["drug"] for d in subset],
+            customdata=[[d["patients"], d["directory"], d["avg_days"], d["cost_pp_pa"]] for d in subset],
+            hovertemplate=(
+                "<b>%{text}</b><br>"
+                "Directory: %{customdata[1]}<br>"
+                "Avg duration: %{customdata[2]} days<br>"
+                "Cost p.a.: £%{customdata[3]:,.0f}<br>"
+                "Patients: %{customdata[0]:,}<br>"
+                "<extra></extra>"
+            ),
+        ))
+
+    # Quadrant lines at median values
+    all_days = [d["avg_days"] for d in data]
+    all_costs = [d["cost_pp_pa"] for d in data]
+    med_days = statistics.median(all_days)
+    med_cost = statistics.median(all_costs)
+
+    fig.add_hline(
+        y=med_cost, line_dash="dash", line_color=ANNOTATION_COLOR,
+        line_width=1,
+        annotation_text=f"Median £{med_cost:,.0f}",
+        annotation_position="top left",
+        annotation_font=dict(size=10, color=ANNOTATION_COLOR, family=CHART_FONT_FAMILY),
+    )
+    fig.add_vline(
+        x=med_days, line_dash="dash", line_color=ANNOTATION_COLOR,
+        line_width=1,
+        annotation_text=f"Median {med_days:.0f} days",
+        annotation_position="top right",
+        annotation_font=dict(size=10, color=ANNOTATION_COLOR, family=CHART_FONT_FAMILY),
+    )
+
+    n_dirs = len(directories)
+    legend = _smart_legend(n_dirs, "Directory")
+    legend_margins = _smart_legend_margin(n_dirs)
+
+    layout = _base_layout(display_title)
+    layout.update(
+        margin=dict(t=60, l=8, **legend_margins),
+        xaxis=dict(
+            title="Average Treatment Duration (days)",
+            gridcolor=GRID_COLOR,
+            zeroline=False,
+        ),
+        yaxis=dict(
+            title="Cost per Patient per Annum (£)",
+            gridcolor=GRID_COLOR,
+            automargin=True,
+            zeroline=False,
+        ),
+        legend=legend,
+        height=550,
+    )
+    fig.update_layout(**layout)
+
+    return fig