fix: adaptive legends + _base_layout for 4 chart functions (Task A.3)

- Add _smart_legend(n_items) and _smart_legend_margin(n_items) helpers
  - >15 items: vertical right legend with extra right margin
  - <=15 items: horizontal legend with dynamic bottom margin
- Apply _base_layout() + _smart_legend() to:
  - create_market_share_figure() — DRUG_PALETTE, adaptive legend
  - create_trust_market_share_figure() — DRUG_PALETTE, adaptive legend
  - create_dosing_figure() — DRUG_PALETTE, legend adapts to trace count
  - create_trust_duration_figure() — TRUST_PALETTE, fixed l=200 margin
- Replace all local nhs_colours lists with module-level palettes
- Net reduction of 48 lines via DRY layout code

IMPLEMENTATION_PLAN.md

@@ -66,23 +66,24 @@ Comprehensive review and improvement of all Plotly charts in the Dash dashboard.
 - **Checkpoint**: Heatmaps show linear color gradient, cell text visible, no fixed width overflow
 
 ### A.3 Fix legend overflow in 4 charts
-- [ ] Create `_smart_legend(n_items)` helper that returns legend dict:
+- [x] Create `_smart_legend(n_items)` helper that returns legend dict:
   - When >15 items: vertical legend on right (`orientation="v", x=1.02, y=1, xanchor="left"`) with dynamic right margin
   - When ≤15: horizontal legend with dynamic bottom margin based on estimated row count
-- [ ] Apply to `create_market_share_figure()` (~L350)
+- [x] Also created `_smart_legend_margin(n_items)` helper returning margin dict with dynamic b/r values
-- [ ] Apply to `create_trust_market_share_figure()` (~L1564)
+- [x] Apply to `create_market_share_figure()` — also replaced local nhs_colours with DRUG_PALETTE
-- [ ] Apply to `create_dosing_figure()` (~L913)
+- [x] Apply to `create_trust_market_share_figure()` — also replaced local nhs_colours with DRUG_PALETTE, fixed Unicode escapes to literal chars
-- [ ] Apply to `create_trust_duration_figure()` (~L1771)
+- [x] Apply to `create_dosing_figure()` — replaced local nhs_colours with DRUG_PALETTE, legend adapts to trace count
-- [ ] Apply `_base_layout()` to all 4 functions
+- [x] Apply to `create_trust_duration_figure()` — replaced local nhs_colours with TRUST_PALETTE, fixed l=200→l=8+automargin
+- [x] Apply `_base_layout()` to all 4 functions
 - **Checkpoint**: Legends don't overlap chart content with 42 drugs or 7 trusts
 
 ### A.4 Fix trust comparison color differentiation
-- [ ] In `create_trust_duration_figure()`: replace `nhs_colours` list with `TRUST_PALETTE`
+- [x] In `create_trust_duration_figure()`: replace `nhs_colours` list with `TRUST_PALETTE` (done in A.3)
 - [ ] Add `is_trust_comparison=False` param to `create_cost_waterfall_figure()` — use `TRUST_PALETTE` when True
 - [ ] Update `tc_cost_waterfall` callback in `dash_app/callbacks/trust_comparison.py` (~L165) to pass `is_trust_comparison=True`
 - [ ] Fix `_dosing_by_drug()` blue→blue interpolation: replace with `plotly.colors.sample_colorscale("Viridis", ...)` for meaningful gradient
-- [ ] Replace `nhs_colours` in `create_trust_market_share_figure()` with `DRUG_PALETTE` for drug traces
+- [x] Replace `nhs_colours` in `create_trust_market_share_figure()` with `DRUG_PALETTE` for drug traces (done in A.3)
-- [ ] Apply `_base_layout()` to all affected functions
+- [x] Apply `_base_layout()` to all affected functions (done in A.3 for trust_market_share and trust_duration)
 - **Checkpoint**: Trust Comparison charts have 7 visually distinct trust colors; dosing has meaningful gradient
 
 ---

src/visualization/plotly_generator.py

@@ -46,6 +46,56 @@ DRUG_PALETTE = [
 ]
 
 
+def _smart_legend(n_items: int, legend_title: str = "") -> dict:
+    """Return a legend dict that adapts to the number of items.
+
+    - >15 items: vertical legend to the right of the chart
+    - ≤15 items: horizontal legend below the chart with dynamic bottom margin
+
+    Returns a dict suitable for ``legend=...`` inside ``fig.update_layout()``.
+    The caller should also set bottom margin accordingly — use
+    ``_smart_legend_margin_b(n_items)`` for that.
+    """
+    base = dict(
+        font=dict(family=CHART_FONT_FAMILY, size=11),
+    )
+    if legend_title:
+        base["title"] = legend_title
+
+    if n_items > 15:
+        base.update(
+            orientation="v",
+            x=1.02,
+            y=1,
+            xanchor="left",
+            yanchor="top",
+        )
+    else:
+        base.update(
+            orientation="h",
+            yanchor="top",
+            y=-0.12,
+            xanchor="center",
+            x=0.5,
+        )
+    return base
+
+
+def _smart_legend_margin(n_items: int) -> dict:
+    """Return margin dict with bottom margin adapted to legend size.
+
+    - >15 items: vertical right legend needs extra right margin (r=140)
+      but minimal bottom margin (b=40).
+    - ≤15 items: horizontal legend needs bottom margin scaled to
+      estimated row count (~6 items per row at font size 11).
+    """
+    if n_items > 15:
+        return dict(r=140, b=40)
+    else:
+        rows = max(1, (n_items + 5) // 6)  # ~6 items per row
+        return dict(b=max(60, rows * 28 + 30), r=24)
+
+
 def _base_layout(title: str, **overrides) -> dict:
     """Return a dict of shared Plotly layout properties.
 
@@ -321,13 +371,6 @@ def create_market_share_figure(data: list[dict], title: str = "") -> go.Figure:
     if not data:
         return go.Figure()
 
-    # NHS blue palette for different drugs
-    nhs_colours = [
-        "#003087", "#005EB8", "#0072CE", "#1E88E5", "#41B6E6",
-        "#4FC3F7", "#768692", "#AE2573", "#006747", "#ED8B00",
-        "#8A1538", "#330072", "#009639", "#DA291C", "#00A499",
-    ]
-
     # Collect unique directorates (in order — already sorted by total patients desc)
     seen_dirs = []
     for d in data:
@@ -341,7 +384,7 @@ def create_market_share_figure(data: list[dict], title: str = "") -> go.Figure:
             seen_drugs.append(d["drug"])
 
     # Build one trace per drug
-    drug_colour_map = {drug: nhs_colours[i % len(nhs_colours)] for i, drug in enumerate(seen_drugs)}
+    drug_colour_map = {drug: DRUG_PALETTE[i % len(DRUG_PALETTE)] for i, drug in enumerate(seen_drugs)}
 
     # Build a lookup: (directory, drug) -> row
     lookup = {(d["directory"], d["drug"]): d for d in data}
@@ -384,60 +427,26 @@ def create_market_share_figure(data: list[dict], title: str = "") -> go.Figure:
 
     display_title = f"First-Line Drug Market Share — {title}" if title else "First-Line Drug Market Share"
 
+    n_drugs = len(seen_drugs)
+    legend_margins = _smart_legend_margin(n_drugs)
+
     fig = go.Figure(data=traces)
-    fig.update_layout(
+    layout = _base_layout(display_title)
+    layout.update(
         barmode="stack",
-        title=dict(
-            text=display_title,
-            font=dict(
-                family="Source Sans 3, system-ui, sans-serif",
-                size=18,
-                color="#1E293B",
-            ),
-            x=0.5,
-            xanchor="center",
-        ),
         xaxis=dict(
             title="% of patients",
             ticksuffix="%",
             range=[0, 105],
-            gridcolor="#E2E8F0",
+            gridcolor=GRID_COLOR,
             zeroline=False,
         ),
-        yaxis=dict(
+        yaxis=dict(title="", automargin=True),
-            title="",
+        legend=_smart_legend(n_drugs, legend_title="Drug"),
-            automargin=True,
+        margin=dict(t=50, l=8, **legend_margins),
-        ),
-        legend=dict(
-            title="Drug",
-            orientation="h",
-            yanchor="top",
-            y=-0.15,
-            xanchor="center",
-            x=0.5,
-            font=dict(
-                family="Source Sans 3, system-ui, sans-serif",
-                size=11,
-            ),
-        ),
-        margin=dict(t=50, l=8, r=24, b=100),
-        paper_bgcolor="rgba(0,0,0,0)",
-        plot_bgcolor="rgba(0,0,0,0)",
-        autosize=True,
-        hoverlabel=dict(
-            bgcolor="#FFFFFF",
-            bordercolor="#CBD5E1",
-            font=dict(
-                family="Source Sans 3, system-ui, sans-serif",
-                size=13,
-                color="#1E293B",
-            ),
-        ),
-        font=dict(
-            family="Source Sans 3, system-ui, sans-serif",
-        ),
         height=max(400, len(seen_dirs) * 60 + 200),
     )
+    fig.update_layout(**layout)
 
     return fig
 
@@ -919,36 +928,22 @@ def create_dosing_figure(
     if not data:
         return go.Figure()
 
-    nhs_colours = [
-        "#005EB8", "#003087", "#41B6E6", "#0066CC", "#1E88E5",
-        "#4FC3F7", "#009639", "#ED8B00", "#768692", "#AE2573",
-        "#8A1538", "#330072", "#DA291C", "#00A499", "#425563",
-    ]
-
     if group_by == "trust":
-        # Single-drug mode: compare trusts, group bars by directory
+        fig = _dosing_by_trust(data, DRUG_PALETTE)
-        fig = _dosing_by_trust(data, nhs_colours)
+        chart_title = "Dosing Intervals by Trust"
-        chart_title = f"Dosing Intervals by Trust"
     else:
-        # Overview mode: weighted average per drug
+        fig = _dosing_by_drug(data, DRUG_PALETTE)
-        fig = _dosing_by_drug(data, nhs_colours)
         chart_title = "Dosing Interval Overview"
 
     if title:
         chart_title = f"{chart_title} — {title}"
 
     n_rows = len(fig.data[0].y) if fig.data else 10
-    fig.update_layout(
+    n_legend = sum(1 for t in fig.data if t.showlegend is not False)
-        title=dict(
+    legend_margins = _smart_legend_margin(n_legend)
-            text=chart_title,
+
-            font=dict(
+    layout = _base_layout(chart_title)
-                family="Source Sans 3, system-ui, sans-serif",
+    layout.update(
-                size=18,
-                color="#003087",
-            ),
-            x=0.5,
-            xanchor="center",
-        ),
         xaxis=dict(
             title="Weekly Interval (weeks between doses)",
             titlefont=dict(size=13, color="#425563"),
@@ -956,30 +951,15 @@ def create_dosing_figure(
             zeroline=True,
             zerolinecolor="rgba(66,85,99,0.2)",
         ),
-        yaxis=dict(
+        yaxis=dict(automargin=True, tickfont=dict(size=11)),
-            automargin=True,
+        margin=dict(t=60, l=20, **legend_margins),
-            tickfont=dict(size=11),
-        ),
-        font=dict(
-            family="Source Sans 3, system-ui, sans-serif",
-            size=12,
-        ),
-        paper_bgcolor="rgba(0,0,0,0)",
-        plot_bgcolor="rgba(0,0,0,0)",
-        margin=dict(t=60, l=20, r=40, b=60),
         height=max(450, n_rows * 40 + 150),
         bargap=0.15,
         bargroupgap=0.05,
         showlegend=True,
-        legend=dict(
+        legend=_smart_legend(n_legend),
-            orientation="h",
-            yanchor="top",
-            y=-0.12,
-            xanchor="center",
-            x=0.5,
-            font=dict(size=11),
-        ),
     )
+    fig.update_layout(**layout)
 
     return fig
 
@@ -1571,12 +1551,6 @@ def create_trust_market_share_figure(
     if not data:
         return go.Figure()
 
-    nhs_colours = [
-        "#003087", "#005EB8", "#0072CE", "#1E88E5", "#41B6E6",
-        "#4FC3F7", "#768692", "#AE2573", "#006747", "#ED8B00",
-        "#8A1538", "#330072", "#009639", "#DA291C", "#00A499",
-    ]
-
     seen_trusts = []
     for d in data:
         t = d["trust_name"]
@@ -1588,7 +1562,7 @@ def create_trust_market_share_figure(
         if d["drug"] not in seen_drugs:
             seen_drugs.append(d["drug"])
 
-    drug_colour_map = {drug: nhs_colours[i % len(nhs_colours)] for i, drug in enumerate(seen_drugs)}
+    drug_colour_map = {drug: DRUG_PALETTE[i % len(DRUG_PALETTE)] for i, drug in enumerate(seen_drugs)}
     lookup = {(d["trust_name"], d["drug"]): d for d in data}
 
     def short_trust(name):
@@ -1611,8 +1585,8 @@ def create_trust_market_share_figure(
                     f"{short_trust(trust)}<br>"
                     f"Patients: {row['patients']:,}<br>"
                     f"Share: {row['proportion']:.1%}<br>"
-                    f"Cost: \u00a3{row['cost']:,.0f}<br>"
+                    f"Cost: £{row['cost']:,.0f}<br>"
-                    f"Cost p.p.p.a: \u00a3{row['cost_pp_pa']:,.0f}"
+                    f"Cost p.p.p.a: £{row['cost_pp_pa']:,.0f}"
                 )
             else:
                 x_vals.append(0)
@@ -1625,32 +1599,21 @@ def create_trust_market_share_figure(
             customdata=hover_texts,
         ))
 
-    display_title = f"Drug Market Share by Trust \u2014 {title}" if title else "Drug Market Share by Trust"
+    display_title = f"Drug Market Share by Trust — {title}" if title else "Drug Market Share by Trust"
+    n_drugs = len(seen_drugs)
+    legend_margins = _smart_legend_margin(n_drugs)
 
     fig = go.Figure(data=traces)
-    fig.update_layout(
+    layout = _base_layout(display_title)
+    layout.update(
         barmode="stack",
-        title=dict(
+        xaxis=dict(title="% of patients", ticksuffix="%", range=[0, 105], gridcolor=GRID_COLOR, zeroline=False),
-            text=display_title,
-            font=dict(family="Source Sans 3, system-ui, sans-serif", size=16, color="#1E293B"),
-            x=0.5, xanchor="center",
-        ),
-        xaxis=dict(title="% of patients", ticksuffix="%", range=[0, 105], gridcolor="#E2E8F0", zeroline=False),
         yaxis=dict(title="", automargin=True),
-        legend=dict(
+        legend=_smart_legend(n_drugs, legend_title="Drug"),
-            title="Drug", orientation="h", yanchor="top", y=-0.15,
+        margin=dict(t=50, l=8, **legend_margins),
-            xanchor="center", x=0.5, font=dict(family="Source Sans 3", size=11),
-        ),
-        margin=dict(t=50, l=8, r=24, b=100),
-        paper_bgcolor="rgba(0,0,0,0)", plot_bgcolor="rgba(0,0,0,0)",
-        autosize=True,
-        hoverlabel=dict(
-            bgcolor="#FFFFFF", bordercolor="#CBD5E1",
-            font=dict(family="Source Sans 3, system-ui, sans-serif", size=13, color="#1E293B"),
-        ),
-        font=dict(family="Source Sans 3, system-ui, sans-serif"),
         height=max(300, len(seen_trusts) * 60 + 200),
     )
+    fig.update_layout(**layout)
 
     return fig
 
@@ -1800,11 +1763,6 @@ def create_trust_duration_figure(
     if not data:
         return go.Figure()
 
-    nhs_colours = [
-        "#005EB8", "#003087", "#41B6E6", "#0066CC", "#1E88E5",
-        "#4FC3F7", "#009639", "#ED8B00", "#768692", "#AE2573",
-    ]
-
     seen_drugs = []
     for d in data:
         if d["drug"] not in seen_drugs:
@@ -1819,7 +1777,7 @@ def create_trust_duration_figure(
     def short_trust(name):
         return name.replace(" NHS FOUNDATION TRUST", "").replace(" HOSPITALS", "")
 
-    trust_colour_map = {t: nhs_colours[i % len(nhs_colours)] for i, t in enumerate(seen_trusts)}
+    trust_colour_map = {t: TRUST_PALETTE[i % len(TRUST_PALETTE)] for i, t in enumerate(seen_trusts)}
     lookup = {(d["drug"], d["trust_name"]): d for d in data}
 
     display_drugs = list(reversed(seen_drugs))
@@ -1852,30 +1810,24 @@ def create_trust_duration_figure(
             customdata=hover_texts,
         ))
 
-    display_title = f"Treatment Duration by Trust \u2014 {title}" if title else "Treatment Duration by Trust"
+    display_title = f"Treatment Duration by Trust — {title}" if title else "Treatment Duration by Trust"
+    n_trusts = len(seen_trusts)
+    legend_margins = _smart_legend_margin(n_trusts)
 
     fig = go.Figure(data=traces)
-    fig.update_layout(
+    layout = _base_layout(display_title)
+    layout.update(
         barmode="group",
-        title=dict(
-            text=display_title,
-            font=dict(family="Source Sans 3, system-ui, sans-serif", size=16, color="#003087"),
-            x=0.5, xanchor="center",
-        ),
         xaxis=dict(
             title="Average Duration (days)", titlefont=dict(size=13, color="#425563"),
             gridcolor="rgba(0,0,0,0.06)", zeroline=True, zerolinecolor="rgba(0,0,0,0.1)",
         ),
         yaxis=dict(title="", automargin=True, tickfont=dict(size=11, color="#425563")),
-        legend=dict(
+        legend=_smart_legend(n_trusts, legend_title="Trust"),
-            title="Trust", orientation="h", yanchor="top", y=-0.12,
+        margin=dict(t=60, l=8, **legend_margins),
-            xanchor="center", x=0.5, font=dict(size=11),
-        ),
-        plot_bgcolor="rgba(0,0,0,0)", paper_bgcolor="rgba(0,0,0,0)",
-        font=dict(family="Source Sans 3, system-ui, sans-serif"),
-        margin=dict(t=60, l=200, r=40, b=100),
         height=max(350, len(seen_drugs) * 35 + 200),
         bargap=0.15, bargroupgap=0.05,
     )
+    fig.update_layout(**layout)
 
     return fig