diff --git a/app/models.py b/app/models.py
index 026aad6..c60ec6b 100644
--- a/app/models.py
+++ b/app/models.py
@@ -128,3 +128,24 @@ class AlertEvent(Base):
     acknowledged_at = Column(DateTime, nullable=True)
     acknowledged_by = Column(String, nullable=True)
     notes = Column(Text, nullable=True)
+
+
+class NL43Reading(Base):
+    """Downsampled time-series of live-monitor readings, for the live-chart
+    backfill (so a viewer sees recent trend on open, not a blank chart).
+
+    Viewing only — NOT the report source. Reports use the device's authoritative
+    FTP .rnd intervals. This is a short, capped trail (one row/minute, pruned to
+    a retention window) fed by the monitor's keepalive poll loop.
+    """
+
+    __tablename__ = "nl43_readings"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    unit_id = Column(String, index=True, nullable=False)
+    timestamp = Column(DateTime, default=func.now(), index=True)
+    lp = Column(String, nullable=True)
+    leq = Column(String, nullable=True)
+    lmax = Column(String, nullable=True)
+    ln1 = Column(String, nullable=True)
+    ln2 = Column(String, nullable=True)
diff --git a/app/monitor.py b/app/monitor.py
index 5f7c8b3..a03dca5 100644
--- a/app/monitor.py
+++ b/app/monitor.py
@@ -36,6 +36,12 @@ MONITOR_POLL_INTERVAL = float(os.getenv("MONITOR_POLL_INTERVAL", "0.25"))
 # per-update latency (~2.5s -> ~1.3s).
 MONITOR_STATE_REFRESH_S = float(os.getenv("MONITOR_STATE_REFRESH_S", "30"))
 
+# Downsampled trail for the live-chart backfill: store one reading per
+# TRAIL_SAMPLE_S and keep TRAIL_RETENTION_HOURS of it (pruned). Viewing only —
+# reports use the device's FTP .rnd data, not this.
+TRAIL_SAMPLE_S = float(os.getenv("MONITOR_TRAIL_SAMPLE_S", "60"))
+TRAIL_RETENTION_HOURS = float(os.getenv("MONITOR_TRAIL_RETENTION_HOURS", "24"))
+
 # If nothing has been broadcast in this many seconds (e.g. device offline and
 # silent), send a keepalive frame so reverse proxies don't drop the idle WS.
 MONITOR_HEARTBEAT_S = float(os.getenv("MONITOR_HEARTBEAT_S", "25"))
@@ -77,6 +83,7 @@ class DeviceMonitor:
         self._reachable = True  # last broadcast reachability (for transition frames)
         self._cached_state: Optional[str] = None  # run state, refreshed periodically
         self._last_state_refresh = 0.0
+        self._last_trail_store = 0.0  # downsample throttle for the backfill trail
 
     @property
     def running(self) -> bool:
@@ -190,6 +197,10 @@ class DeviceMonitor:
             snap.unit_id = self.unit_id
             persist_snapshot(snap, db)
             db.commit()
+            # Append to the downsampled backfill trail (~one row per TRAIL_SAMPLE_S).
+            if now - self._last_trail_store >= TRAIL_SAMPLE_S:
+                self._last_trail_store = now
+                self._store_trail(snap, db)
             status = db.query(NL43Status).filter_by(unit_id=self.unit_id).first()
             mst = (status.measurement_start_time.isoformat()
                    if status and status.measurement_start_time else None)
@@ -200,6 +211,24 @@ class DeviceMonitor:
         finally:
             db.close()
 
+    def _store_trail(self, snap, db) -> None:
+        """Append one downsampled reading to the backfill trail and prune old rows."""
+        from datetime import datetime, timedelta
+        from app.models import NL43Reading
+        try:
+            db.add(NL43Reading(
+                unit_id=self.unit_id, timestamp=datetime.utcnow(),
+                lp=snap.lp, leq=snap.leq, lmax=snap.lmax, ln1=snap.ln1, ln2=snap.ln2,
+            ))
+            cutoff = datetime.utcnow() - timedelta(hours=TRAIL_RETENTION_HOURS)
+            db.query(NL43Reading).filter(
+                NL43Reading.unit_id == self.unit_id,
+                NL43Reading.timestamp < cutoff,
+            ).delete()
+            db.commit()
+        except Exception as e:
+            logger.warning(f"[MONITOR] {self.unit_id}: trail store failed: {e}")
+
     def _broadcast(self, payload: dict, cache: bool = True) -> None:
         if cache:
             self._last_payload = payload  # replayed to new subscribers
diff --git a/app/routers.py b/app/routers.py
index 80aeaaa..aae9a23 100644
--- a/app/routers.py
+++ b/app/routers.py
@@ -11,7 +11,7 @@ import os
 import asyncio
 
 from app.database import get_db
-from app.models import NL43Config, NL43Status, AlertRule, AlertEvent
+from app.models import NL43Config, NL43Status, AlertRule, AlertEvent, NL43Reading
 from app.services import NL43Client, persist_snapshot
 
 logger = logging.getLogger(__name__)
@@ -330,6 +330,31 @@ async def monitor_status():
     return {"status": "ok", "monitors": monitor_manager.status()}
 
 
+@router.get("/{unit_id}/history")
+def get_monitor_history(unit_id: str, hours: float = 2.0, db: Session = Depends(get_db)):
+    """Recent downsampled monitor readings (the DOD trail) for the live-chart
+    backfill. Viewing only — NOT the FTP report data."""
+    from datetime import timedelta
+    hours = max(0.1, min(hours, 48.0))
+    cutoff = datetime.utcnow() - timedelta(hours=hours)
+    rows = (db.query(NL43Reading)
+            .filter(NL43Reading.unit_id == unit_id, NL43Reading.timestamp >= cutoff)
+            .order_by(NL43Reading.timestamp.asc()).all())
+    return {
+        "status": "ok",
+        "unit_id": unit_id,
+        "hours": hours,
+        "count": len(rows),
+        "readings": [
+            {
+                "timestamp": r.timestamp.isoformat() if r.timestamp else None,
+                "lp": r.lp, "leq": r.leq, "lmax": r.lmax, "ln1": r.ln1, "ln2": r.ln2,
+            }
+            for r in rows
+        ],
+    }
+
+
 # ============================================================================
 # ALERTS — threshold rules + fired events
 # ============================================================================