diff --git a/pkg/metricstore/buffer.go b/pkg/metricstore/buffer.go
index 69b401f3..1205356d 100644
--- a/pkg/metricstore/buffer.go
+++ b/pkg/metricstore/buffer.go
@@ -303,43 +303,134 @@ func (b *buffer) firstWrite() int64 {
 //   - error:          Non-nil on failure
 //
 // Panics if 'data' slice is too small to hold all values in [from, to).
-func (b *buffer) read(from, to int64, data []schema.Float) ([]schema.Float, int64, int64, error) {
-	// Walk back to the buffer that covers 'from', adjusting if we hit the oldest.
-	for from < b.firstWrite() {
-		if b.prev == nil {
-			from = b.firstWrite()
-			break
-		}
+// func (b *buffer) read(from, to int64, data []schema.Float) ([]schema.Float, int64, int64, error) {
+// 	// Walk back to the buffer that covers 'from', adjusting if we hit the oldest.
+// 	for from < b.firstWrite() {
+// 		if b.prev == nil {
+// 			from = b.firstWrite()
+// 			break
+// 		}
+// 		b = b.prev
+// 	}
+
+// 	i := 0
+// 	t := from
+// 	for ; t < to; t += b.frequency {
+// 		idx := int((t - b.start) / b.frequency)
+// 		if idx >= cap(b.data) {
+// 			if b.next == nil {
+// 				break
+// 			}
+// 			b = b.next
+// 			// Recalculate idx in the new buffer; a gap between buffers may exist.
+// 			idx = int((t - b.start) / b.frequency)
+// 		}
+
+// 		if idx >= len(b.data) {
+// 			if b.next == nil || to <= b.next.start {
+// 				break
+// 			}
+// 			data[i] += schema.NaN // NaN + anything = NaN; propagates missing data
+// 		} else if t < b.start {
+// 			data[i] += schema.NaN // gap before this buffer's first write
+// 		} else {
+// 			data[i] += b.data[idx]
+// 		}
+// 		i++
+// 	}
+
+// 	return data[:i], from, t, nil
+// }
+
+// read retrieves time-series data from the buffer chain for the specified time range.
+//
+// Traverses the buffer chain backwards (via prev links) if 'from' precedes the current
+// buffer's start. Missing data points are represented as NaN. Values are accumulated
+// into the provided 'data' slice (using +=, so caller must zero-initialize if needed).
+//
+// Points with no stored data (before the oldest write, in inter-buffer gaps, or
+// past the newest write) are NaN-filled while scanning. Whether those NaN pads
+// are kept is controlled by 'trim':
+//
+//   - trim == false: the returned slice always spans the full requested window
+//     [from, to), NaN where missing. Use this when several scopes accumulate
+//     into one index-aligned slice (aggregation) or when the caller wants the
+//     whole window for display.
+//   - trim == true: the returned slice is cut down to the real data extent
+//     [dataFrom, dataTo), i.e. leading and trailing NaN pads are dropped so that
+//     result[0] aligns with dataFrom. Use this when persisting only real data
+//     (checkpointing).
+//
+// The second and third return values report the actual extent of real (non-NaN)
+// data found within the window: dataFrom is the timestamp of the first stored
+// sample and dataTo is one frequency past the last. When the buffer fully covers
+// the request these equal from/to; when a scope holds less data they shrink,
+// which is how callers detect misalignment across aggregated scopes. If no real
+// data exists in the window both equal 'from' (and a trimmed read returns an
+// empty slice).
+//
+// Parameters:
+//   - from: Start timestamp (Unix seconds)
+//   - to:   End timestamp (Unix seconds, exclusive)
+//   - data: Pre-allocated slice to accumulate results (must be large enough)
+//   - trim: If true, cut the result to the real data extent; if false, keep the
+//     full NaN-padded [from, to) window.
+//
+// Returns:
+//   - []schema.Float: Full [from, to) window, or the [dataFrom, dataTo) slice if trim
+//   - int64:          dataFrom — timestamp of the first real sample in the window
+//   - int64:          dataTo   — one frequency past the last real sample
+//   - error:          Non-nil on failure
+//
+// Panics if 'data' slice is too small to hold all values in [from, to).
+func (b *buffer) read(from, to int64, data []schema.Float, trim bool) ([]schema.Float, int64, int64, error) {
+	// Walk back toward the buffer that covers 'from'. Do NOT clamp 'from'
+	// forward when it predates the oldest write; those leading points are
+	// NaN-filled below so the full requested window is always available.
+	for from < b.firstWrite() && b.prev != nil {
 		b = b.prev
 	}
 
+	freq := b.frequency // constant across the chain for this metric level
 	i := 0
 	t := from
+	dataFrom, dataTo := from, from // real (non-NaN) data extent within [from, to)
+	haveData := false
 	for ; t < to; t += b.frequency {
 		idx := int((t - b.start) / b.frequency)
 		if idx >= cap(b.data) {
 			if b.next == nil {
-				break
+				data[i] += schema.NaN // past the newest buffer; rest is missing
+				i++
+				continue
 			}
 			b = b.next
 			// Recalculate idx in the new buffer; a gap between buffers may exist.
 			idx = int((t - b.start) / b.frequency)
 		}
 
-		if idx >= len(b.data) {
-			if b.next == nil || to <= b.next.start {
-				break
-			}
+		if idx >= len(b.data) || t < b.start {
+			// Not yet written, or in the gap before this buffer's first write.
 			data[i] += schema.NaN // NaN + anything = NaN; propagates missing data
-		} else if t < b.start {
-			data[i] += schema.NaN // gap before this buffer's first write
 		} else {
 			data[i] += b.data[idx]
+			if !haveData {
+				dataFrom = t
+				haveData = true
+			}
+			dataTo = t + b.frequency
 		}
 		i++
 	}
 
-	return data[:i], from, t, nil
+	if trim {
+		// Drop leading/trailing NaN pads so result[0] aligns with dataFrom.
+		front := int((dataFrom - from) / freq)
+		back := int((dataTo - from) / freq)
+		return data[front:back], dataFrom, dataTo, nil
+	}
+
+	return data[:i], dataFrom, dataTo, nil
 }
 
 // free removes buffers older than the specified timestamp from the chain.
diff --git a/pkg/metricstore/checkpoint.go b/pkg/metricstore/checkpoint.go
index 48da842d..3e3032d8 100644
--- a/pkg/metricstore/checkpoint.go
+++ b/pkg/metricstore/checkpoint.go
@@ -298,15 +298,14 @@ func (l *Level) toCheckpointFile(from, to int64, m *MemoryStore) (*CheckpointFil
 		}
 
 		data := make([]schema.Float, (to-from)/b.frequency+1)
-		data, start, end, err := b.read(from, to, data)
+
+		// trim=true: read() returns only the real data extent, sliced so that
+		// Data[0] aligns with Start (the checkpoint reload contract).
+		data, start, _, err := b.read(from, to, data, true)
 		if err != nil {
 			return nil, err
 		}
 
-		for i := int((end - start) / b.frequency); i < len(data); i++ {
-			data[i] = schema.NaN
-		}
-
 		retval.Metrics[metric] = &CheckpointMetrics{
 			Frequency: b.frequency,
 			Start:     start,
diff --git a/pkg/metricstore/metricstore.go b/pkg/metricstore/metricstore.go
index 6cf2dab9..039245e3 100644
--- a/pkg/metricstore/metricstore.go
+++ b/pkg/metricstore/metricstore.go
@@ -708,32 +708,36 @@ func (m *MemoryStore) Read(selector util.Selector, metric string, from, to, reso
 		return nil, 0, 0, 0, errors.New("[METRICSTORE]> unknown metric: " + metric)
 	}
 
+	// data spans the full requested window; every scope's read() writes into the
+	// same index-aligned slice (NaN where it has no value), so aggregation never
+	// needs trimming. dataFrom/dataTo track the real (non-NaN) extent of the first
+	// scope seen; later scopes that report a different extent are misaligned. We
+	// no longer abort on misalignment — the full NaN-padded window is still
+	// returned for display — but we log it so the condition stays visible.
 	n, data := 0, make([]schema.Float, (to-from)/minfo.Frequency+1)
+	var dataFrom, dataTo int64
 
 	err := m.root.findBuffers(selector, minfo.offset, func(b *buffer) error {
-		cdata, cfrom, cto, err := b.read(from, to, data)
+		cdata, cfrom, cto, err := b.read(from, to, data, false)
 		if err != nil {
 			return err
 		}
 
 		if n == 0 {
-			from, to = cfrom, cto
-		} else if from != cfrom || to != cto || len(data) != len(cdata) {
-			missingfront, missingback := int((from-cfrom)/minfo.Frequency), int((to-cto)/minfo.Frequency)
-			if missingfront != 0 {
-				return ErrDataDoesNotAlignMissingFront
+			dataFrom, dataTo = cfrom, cto
+		} else if cfrom != dataFrom || cto != dataTo {
+			missingfront, missingback := int((dataFrom-cfrom)/minfo.Frequency), int((dataTo-cto)/minfo.Frequency)
+			switch {
+			case missingfront != 0:
+				cclog.Warnf("%s", fmt.Errorf("%w: metric=%s buf#%d freq=%d ref[%d,%d] this[%d,%d] missingfront=%d pts",
+					ErrDataDoesNotAlignMissingFront, metric, n, minfo.Frequency, dataFrom, dataTo, cfrom, cto, missingfront))
+			case missingback != 0:
+				cclog.Warnf("%s", fmt.Errorf("%w: metric=%s buf#%d freq=%d ref[%d,%d] this[%d,%d] missingback=%d pts",
+					ErrDataDoesNotAlignMissingBack, metric, n, minfo.Frequency, dataFrom, dataTo, cfrom, cto, missingback))
+			default:
+				cclog.Warnf("%s", fmt.Errorf("%w: metric=%s buf#%d ref[%d,%d] this[%d,%d]",
+					ErrDataDoesNotAlignDataLenMismatch, metric, n, dataFrom, dataTo, cfrom, cto))
 			}
-
-			newlen := len(cdata) - missingback
-			if newlen < 1 {
-				return ErrDataDoesNotAlignMissingBack
-			}
-			cdata = cdata[0:newlen]
-			if len(cdata) != len(data) {
-				return ErrDataDoesNotAlignDataLenMismatch
-			}
-
-			from, to = cfrom, cto
 		}
 
 		data = cdata
diff --git a/pkg/metricstore/metricstore_test.go b/pkg/metricstore/metricstore_test.go
index 35f97278..1743e5d4 100644
--- a/pkg/metricstore/metricstore_test.go
+++ b/pkg/metricstore/metricstore_test.go
@@ -141,23 +141,35 @@ func TestBufferWriteOverwrite(t *testing.T) {
 
 // ─── Buffer read ──────────────────────────────────────────────────────────────
 
-// TestBufferReadBeforeFirstWrite verifies that 'from' is clamped to firstWrite
-// when the requested range starts before any data in the chain.
+// TestBufferReadBeforeFirstWrite verifies that a range starting before any data
+// in the chain returns the full requested window with the leading, unwritten
+// points NaN-filled, and reports the real data extent (dataFrom) via the second
+// return value (rather than clamping 'from' forward and truncating).
 func TestBufferReadBeforeFirstWrite(t *testing.T) {
 	b := newBuffer(100, 10) // firstWrite = 100
 	b.write(100, schema.Float(1.0))
 	b.write(110, schema.Float(2.0))
 
 	data := make([]schema.Float, 10)
-	result, adjustedFrom, _, err := b.read(50, 120, data)
+	result, dataFrom, dataTo, err := b.read(50, 120, data, false)
 	if err != nil {
 		t.Fatalf("read() error = %v", err)
 	}
-	if adjustedFrom != 100 {
-		t.Errorf("adjustedFrom = %d, want 100 (clamped to firstWrite)", adjustedFrom)
+	// Real data extent, not the requested window: first sample at 100, last at 110.
+	if dataFrom != 100 || dataTo != 120 {
+		t.Errorf("dataFrom/dataTo = %d/%d, want 100/120", dataFrom, dataTo)
 	}
-	if len(result) != 2 {
-		t.Errorf("len(result) = %d, want 2", len(result))
+	// Full window [50,120) step 10 = 7 points; first 5 predate firstWrite=100 -> NaN.
+	if len(result) != 7 {
+		t.Fatalf("len(result) = %d, want 7", len(result))
+	}
+	for i := 0; i < 5; i++ {
+		if !result[i].IsNaN() {
+			t.Errorf("result[%d] = %v, want NaN", i, result[i])
+		}
+	}
+	if result[5] != 1.0 || result[6] != 2.0 {
+		t.Errorf("result[5:7] = %v, want [1 2]", result[5:7])
 	}
 }
 
@@ -177,7 +189,7 @@ func TestBufferReadChain(t *testing.T) {
 	b1.next = b2
 
 	data := make([]schema.Float, 6)
-	result, from, to, err := b2.read(100, 160, data)
+	result, from, to, err := b2.read(100, 160, data, false)
 	if err != nil {
 		t.Fatalf("read() error = %v", err)
 	}
@@ -219,7 +231,7 @@ func TestBufferReadIdxAfterSwitch(t *testing.T) {
 	// from=0 triggers the walkback to b1 (from < b2.firstWrite=5).
 	// After clamping, the loop runs t=5,15,25,35.
 	data := make([]schema.Float, 4)
-	result, _, _, err := b2.read(0, 36, data)
+	result, _, _, err := b2.read(0, 36, data, false)
 	if err != nil {
 		t.Fatalf("read() error = %v", err)
 	}
@@ -247,7 +259,7 @@ func TestBufferReadNaNValues(t *testing.T) {
 	b.write(120, schema.Float(3.0))
 
 	data := make([]schema.Float, 3)
-	result, _, _, err := b.read(100, 130, data)
+	result, _, _, err := b.read(100, 130, data, false)
 	if err != nil {
 		t.Fatalf("read() error = %v", err)
 	}
@@ -274,7 +286,7 @@ func TestBufferReadAccumulation(t *testing.T) {
 
 	// Pre-populate data slice (simulates a second metric being summed in).
 	data := []schema.Float{2.0, 1.0, 0.0}
-	result, _, _, err := b.read(100, 120, data)
+	result, _, _, err := b.read(100, 120, data, false)
 	if err != nil {
 		t.Fatalf("read() error = %v", err)
 	}
@@ -577,7 +589,7 @@ func TestBufferRead(t *testing.T) {
 
 	// Read data
 	data := make([]schema.Float, 3)
-	result, from, to, err := b.read(100, 130, data)
+	result, from, to, err := b.read(100, 130, data, false)
 	if err != nil {
 		t.Errorf("buffer.read() error = %v", err)
 	}
diff --git a/pkg/metricstore/stats.go b/pkg/metricstore/stats.go
index 58dcdd7d..4cc3d861 100644
--- a/pkg/metricstore/stats.go
+++ b/pkg/metricstore/stats.go
@@ -7,6 +7,7 @@ package metricstore
 
 import (
 	"errors"
+	"fmt"
 	"math"
 
 	"github.com/ClusterCockpit/cc-lib/v2/schema"
@@ -142,9 +143,11 @@ func (m *MemoryStore) Stats(selector util.Selector, metric string, from, to int6
 		if n == 0 {
 			from, to = cfrom, cto
 		} else if from != cfrom {
-			return ErrDataDoesNotAlignMissingFront
+			return fmt.Errorf("%w: metric=%s buf#%d want[%d,%d] got[%d,%d]",
+				ErrDataDoesNotAlignMissingFront, metric, n, from, to, cfrom, cto)
 		} else if to != cto {
-			return ErrDataDoesNotAlignMissingBack
+			return fmt.Errorf("%w: metric=%s buf#%d want[%d,%d] got[%d,%d]",
+				ErrDataDoesNotAlignMissingBack, metric, n, from, to, cfrom, cto)
 		}
 
 		samples += stats.Samples