diff --git a/backend/app.py b/backend/app.py
index 08895be..1d86a6f 100644
--- a/backend/app.py
+++ b/backend/app.py
@@ -676,9 +676,6 @@ async def get_national_averages(request: Request):
     if df.empty:
         return {"primary": {}, "secondary": {}}
 
-    latest_year = int(df["year"].max())
-    df_latest = df[df["year"] == latest_year]
-
     ks2_metrics = [
         "rwm_expected_pct", "rwm_high_pct",
         "reading_expected_pct", "writing_expected_pct", "maths_expected_pct",
@@ -703,15 +700,30 @@ async def get_national_averages(request: Request):
                     out[col] = round(float(val.mean()), 2)
         return out
 
+    latest_year = int(df["year"].max())
+    df_latest = df[df["year"] == latest_year]
+
     # Primary: schools where KS2 data is non-null
     primary_df = df_latest[df_latest["rwm_expected_pct"].notna()]
     # Secondary: schools where KS4 data is non-null
     secondary_df = df_latest[df_latest["attainment_8_score"].notna()]
 
+    # Per-year averages for every year in the dataset (used by chart reference lines)
+    by_year = []
+    for yr in sorted(df["year"].dropna().unique()):
+        yr = int(yr)
+        df_yr = df[df["year"] == yr]
+        by_year.append({
+            "year": yr,
+            "primary": _means(df_yr[df_yr["rwm_expected_pct"].notna()], ks2_metrics),
+            "secondary": _means(df_yr[df_yr["attainment_8_score"].notna()], ks4_metrics),
+        })
+
     return {
         "year": latest_year,
         "primary": _means(primary_df, ks2_metrics),
         "secondary": _means(secondary_df, ks4_metrics),
+        "by_year": by_year,
     }
 
 
diff --git a/nextjs-app/components/PerformanceChart.tsx b/nextjs-app/components/PerformanceChart.tsx
index 300a3f1..4ca0a0f 100644
--- a/nextjs-app/components/PerformanceChart.tsx
+++ b/nextjs-app/components/PerformanceChart.tsx
@@ -24,14 +24,22 @@ import styles from './PerformanceChart.module.css';
 
 ChartJS.register(CategoryScale, LinearScale, PointElement, LineElement, Title, Tooltip, Legend);
 
+interface NationalByYear {
+  year: number;
+  primary: Record<string, number>;
+  secondary: Record<string, number>;
+}
+
 interface PerformanceChartProps {
   data: SchoolResult[];
   schoolName: string;
   isSecondary?: boolean;
-  /** National average RWM expected % — rendered as a dashed reference line */
+  /** National average RWM expected % for the latest year — fallback if no by_year data */
   nationalRwmAvg?: number | null;
-  /** National average Attainment 8 — rendered as a dashed reference line */
+  /** National average Attainment 8 for the latest year — fallback if no by_year data */
   nationalAtt8Avg?: number | null;
+  /** Per-year national averages — used to draw a changing reference line */
+  nationalByYear?: NationalByYear[];
 }
 
 // Academic years when SATs/GCSEs were cancelled due to COVID
@@ -42,10 +50,30 @@ export function PerformanceChart({
   isSecondary = false,
   nationalRwmAvg,
   nationalAtt8Avg,
+  nationalByYear,
 }: PerformanceChartProps) {
   const sortedData = [...data].sort((a, b) => a.year - b.year);
   const years = sortedData.map(d => formatAcademicYear(d.year));
 
+  // Build per-year national average series aligned to the school's data years.
+  // Falls back to a flat line using the scalar prop if by_year isn't available.
+  const natRefRwm: (number | null)[] = sortedData.map(d => {
+    if (nationalByYear) {
+      const match = nationalByYear.find(n => n.year === d.year);
+      return match?.primary?.rwm_expected_pct ?? null;
+    }
+    return nationalRwmAvg ?? null;
+  });
+  const natRefAtt8: (number | null)[] = sortedData.map(d => {
+    if (nationalByYear) {
+      const match = nationalByYear.find(n => n.year === d.year);
+      return match?.secondary?.attainment_8_score ?? null;
+    }
+    return nationalAtt8Avg ?? null;
+  });
+  const hasNatRwm = natRefRwm.some(v => v != null);
+  const hasNatAtt8 = natRefAtt8.some(v => v != null);
+
   // ── Trend summary (primary only) ──────────────────────────────────────
   const trendSummary = (() => {
     if (isSecondary) return null;
@@ -114,10 +142,10 @@ export function PerformanceChart({
       hidden: true,
       yAxisID: 'y1',
     },
-    ...(nationalAtt8Avg != null ? [{
+    ...(hasNatAtt8 ? [{
       ...refLineStyle,
       label: 'National average',
-      data: sortedData.map(() => nationalAtt8Avg!),
+      data: natRefAtt8,
       yAxisID: 'y',
     } as ChartDataset<'line'>] : []),
   ] : [
@@ -142,10 +170,10 @@ export function PerformanceChart({
       pointRadius: 3,
       yAxisID: 'y',
     },
-    ...(nationalRwmAvg != null ? [{
+    ...(hasNatRwm ? [{
       ...refLineStyle,
       label: 'National average',
-      data: sortedData.map(() => nationalRwmAvg!),
+      data: natRefRwm,
       yAxisID: 'y',
     } as ChartDataset<'line'>] : []),
     {
diff --git a/nextjs-app/components/SchoolDetailView.tsx b/nextjs-app/components/SchoolDetailView.tsx
index 99782dd..ce1f150 100644
--- a/nextjs-app/components/SchoolDetailView.tsx
+++ b/nextjs-app/components/SchoolDetailView.tsx
@@ -1030,6 +1030,7 @@ export function SchoolDetailView({
               isSecondary={isSecondary}
               nationalRwmAvg={isPrimary ? (primaryAvg.rwm_expected_pct ?? null) : null}
               nationalAtt8Avg={isSecondary ? (secondaryAvg.attainment_8_score ?? null) : null}
+              nationalByYear={nationalAvg?.by_year}
             />
           </div>
           {yearlyData.length > 1 && (
diff --git a/nextjs-app/components/SecondarySchoolDetailView.tsx b/nextjs-app/components/SecondarySchoolDetailView.tsx
index 7219f82..00e124a 100644
--- a/nextjs-app/components/SecondarySchoolDetailView.tsx
+++ b/nextjs-app/components/SecondarySchoolDetailView.tsx
@@ -618,6 +618,7 @@ export function SecondarySchoolDetailView({
                   schoolName={schoolInfo.school_name}
                   isSecondary={true}
                   nationalAtt8Avg={heroAtt8Nat}
+                  nationalByYear={nationalAvg?.by_year}
                 />
               </div>
             </>
diff --git a/nextjs-app/lib/types.ts b/nextjs-app/lib/types.ts
index 7ead4bf..eddb1b1 100644
--- a/nextjs-app/lib/types.ts
+++ b/nextjs-app/lib/types.ts
@@ -351,12 +351,17 @@ export interface Filters {
   admissions_policies: string[];
 }
 
-export interface NationalAverages {
+export interface NationalAveragesYear {
   year: number;
   primary: Record<string, number>;
   secondary: Record<string, number>;
 }
 
+export interface NationalAverages extends NationalAveragesYear {
+  /** Per-year averages for every year in the dataset, used for chart reference lines. */
+  by_year: NationalAveragesYear[];
+}
+
 export interface LAaveragesResponse {
   year: number;
   secondary: {