feat: pathway depth distribution chart tab (Task C.2)

Horizontal bar chart showing patients who stopped at each treatment
line depth (exclusive counts, not cumulative like the funnel).

dash_app/callbacks/chart.py

@@ -292,6 +292,31 @@ def _render_funnel(app_state, title):
     return create_retention_funnel_figure(data, title)
 
 
+def _render_depth(app_state, title):
+    """Build the pathway depth distribution figure from current filter state."""
+    from dash_app.data.queries import get_pathway_depth_distribution
+    from visualization.plotly_generator import create_pathway_depth_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_pathway_depth_distribution(filter_id, chart_type, directory, trust)
+    except Exception:
+        log.exception("Failed to load pathway depth data")
+        return _empty_figure("Failed to load pathway depth data.")
+
+    if not data:
+        return _empty_figure("No pathway depth data available.\nTry adjusting your filters.")
+
+    return create_pathway_depth_figure(data, title)
+
+
 def register_chart_callbacks(app):
     """Register tab switching, pathway data loading, and chart rendering callbacks."""
 
@@ -435,6 +460,9 @@ def register_chart_callbacks(app):
         elif active_tab == "funnel":
             fig = _render_funnel(app_state, title)
 
+        elif active_tab == "depth":
+            fig = _render_depth(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

@@ -9,6 +9,7 @@ TAB_DEFINITIONS = [
     ("sankey", "Sankey"),
     ("heatmap", "Heatmap"),
     ("funnel", "Funnel"),
+    ("depth", "Depth"),
 ]
 
 # Full set retained for Trust Comparison dashboard (Phase 10.8)

dash_app/data/queries.py

@@ -25,6 +25,7 @@ from data_processing.pathway_queries import (
     get_trust_durations as _get_trust_durations,
     get_directorate_summary as _get_directorate_summary,
     get_retention_funnel as _get_retention_funnel,
+    get_pathway_depth_distribution as _get_pathway_depth_distribution,
 )
 
 DB_PATH = Path(__file__).resolve().parents[2] / "data" / "pathways.db"
@@ -194,3 +195,13 @@ def get_retention_funnel(
 ) -> list[dict]:
     """Patient retention by treatment line depth."""
     return _get_retention_funnel(DB_PATH, date_filter_id, chart_type, directory, trust)
+
+
+def get_pathway_depth_distribution(
+    date_filter_id: str = "all_6mo",
+    chart_type: str = "directory",
+    directory: Optional[str] = None,
+    trust: Optional[str] = None,
+) -> 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)