diff --git a/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/KpiTrendChart.razor.cs b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/KpiTrendChart.razor.cs
index 27f2a2cb..56e19117 100644
--- a/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/KpiTrendChart.razor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/KpiTrendChart.razor.cs
@@ -82,9 +82,6 @@ public partial class KpiTrendChart
     /// <summary>Exactly one sample — show the single-sample note, not a polyline.</summary>
     private bool HasSingleSample => IsAvailable && Series.Count == 1;
 
-    /// <summary>Available + ≥1 point but not a full chart, or unavailable / empty.</summary>
-    private bool ShowPlaceholder => !IsAvailable || Series.Count == 0;
-
     /// <summary>
     /// Stable Playwright hook: <c>kpi-trend-&lt;slug&gt;</c> where the slug is the
     /// title lowercased with each run of non-alphanumerics collapsed to a single
@@ -175,13 +172,17 @@ public partial class KpiTrendChart
             var p = Series[i];
 
             // X — by time fraction, or even index spacing when all timestamps equal.
+            // n >= 2 is guaranteed by the HasChart precondition (Series.Count >= 2),
+            // so the (n - 1) divisor is always ≥ 1 and the n == 1 arm is unreachable.
             double xFrac = timeSpanTicks > 0
                 ? (p.BucketStartUtc.Ticks - firstTicks) / timeSpanTicks
-                : (n == 1 ? 0.0 : (double)i / (n - 1));
+                : (double)i / (n - 1);
             var x = PadX + (xFrac * plotW);
 
             // Y — baseline at 0, top at max. Flat at baseline when max == 0.
-            double yFrac = max > 0 ? p.Value / max : 0.0;
+            // Clamp to non-negative so a stale negative Value cannot push a point
+            // above the baseline or outside the viewBox.
+            double yFrac = max > 0 ? Math.Max(0, p.Value) / max : 0.0;
             var y = baselineY - (yFrac * plotH);
 
             if (i > 0)
diff --git a/src/ZB.MOM.WW.ScadaBridge.Commons/Types/Kpi/KpiSeriesBucketer.cs b/src/ZB.MOM.WW.ScadaBridge.Commons/Types/Kpi/KpiSeriesBucketer.cs
index 78e87ba7..9eb512bd 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Commons/Types/Kpi/KpiSeriesBucketer.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Commons/Types/Kpi/KpiSeriesBucketer.cs
@@ -27,7 +27,8 @@ public static class KpiSeriesBucketer
     ///   An <see cref="IReadOnlyList{T}"/> of at most <paramref name="maxPoints"/> bucketed points,
     ///   ordered by <see cref="KpiSeriesPoint.BucketStartUtc"/> ascending.
     ///   Returns <paramref name="raw"/> unchanged (same reference) when
-    ///   <c>raw.Count &lt;= maxPoints</c>.
+    ///   <c>raw.Count &lt;= maxPoints</c>; callers must not mutate the underlying
+    ///   collection in that case, as it is the same object passed in.
     /// </returns>
     /// <exception cref="ArgumentOutOfRangeException">
     ///   Thrown when <paramref name="maxPoints"/> &lt; 2 or
diff --git a/src/ZB.MOM.WW.ScadaBridge.KpiHistory/KpiHistoryRecorderActor.cs b/src/ZB.MOM.WW.ScadaBridge.KpiHistory/KpiHistoryRecorderActor.cs
index f2f942a0..d85dda0f 100644
--- a/src/ZB.MOM.WW.ScadaBridge.KpiHistory/KpiHistoryRecorderActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.KpiHistory/KpiHistoryRecorderActor.cs
@@ -87,9 +87,9 @@ public class KpiHistoryRecorderActor : ReceiveActor, IWithTimers
         _logger = logger ?? throw new ArgumentNullException(nameof(logger));
 
         Receive<SampleTick>(_ => HandleSampleTick());
-        Receive<SampleComplete>(_ => { });
+        Receive<SampleComplete>(_ => { }); // best-effort: no actor state to reset on completion
         Receive<PurgeTick>(_ => HandlePurgeTick());
-        Receive<PurgeComplete>(_ => { });
+        Receive<PurgeComplete>(_ => { }); // best-effort: no actor state to reset on completion
     }
 
     /// <inheritdoc />
diff --git a/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Components/Shared/KpiTrendChartTests.cs b/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Components/Shared/KpiTrendChartTests.cs
index ca5d14d2..e60a1e23 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Components/Shared/KpiTrendChartTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Components/Shared/KpiTrendChartTests.cs
@@ -106,7 +106,12 @@ public class KpiTrendChartTests : BunitContext
             .Add(c => c.Title, "Flat Time")
             .Add(c => c.IsAvailable, true));
 
-        Assert.Contains("<polyline", cut.Markup);
+        var markup = cut.Markup;
+        Assert.Contains("<polyline", markup);
+        // The even-spacing fallback must produce finite coordinates — no NaN or Infinity
+        // from the divide-by-zero guard path.
+        Assert.DoesNotContain("NaN", markup);
+        Assert.DoesNotContain("Infinity", markup);
     }
 
     [Fact]
@@ -123,7 +128,12 @@ public class KpiTrendChartTests : BunitContext
             .Add(c => c.Title, "Quiet")
             .Add(c => c.IsAvailable, true));
 
-        Assert.Contains("<polyline", cut.Markup);
+        var markup = cut.Markup;
+        Assert.Contains("<polyline", markup);
+        // The zero-max flat-line guard must produce finite coordinates — no NaN or Infinity
+        // from the yFrac divide-by-zero protection path.
+        Assert.DoesNotContain("NaN", markup);
+        Assert.DoesNotContain("Infinity", markup);
     }
 
     [Theory]
diff --git a/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Kpi/KpiSeriesBucketerTests.cs b/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Kpi/KpiSeriesBucketerTests.cs
index 1f43a86c..331cec89 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Kpi/KpiSeriesBucketerTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Kpi/KpiSeriesBucketerTests.cs
@@ -190,33 +190,47 @@ public class KpiSeriesBucketerTests
     [Fact]
     public void Bucket_PointAtToUtc_LandsInLastBucket()
     {
-        // 60-minute window / 3 buckets → 20 min each.
-        // A point exactly at T(60) = toUtc must go to bucket 2, not overflow.
+        // 60-minute window / 2 buckets → 30 min each.
+        // A point exactly at T(60) = toUtc must go to bucket 1 (the last bucket),
+        // not overflow to an out-of-range index.
+        // T(10) → bucket 0: [0, 30); T(35) → bucket 1: [30, 60]; T(60) → bucket 1.
         var raw = new[]
-        {
-            new KpiSeriesPoint(T(10), 1.0),
-            new KpiSeriesPoint(T(30), 2.0),
-            new KpiSeriesPoint(T(60), 99.0), // right edge — must land in last bucket
-        };
-
-        // raw.Count (3) == maxPoints (3) so it normally returns as-is;
-        // use maxPoints=2 to force downsampling and expose the edge behaviour.
-        // Window: [T(0), T(60)], 2 buckets → 30 min each.
-        // T(10) → bucket 0, T(30) → bucket 1, T(60) → bucket 1 (last).
-        var raw2 = new[]
         {
             new KpiSeriesPoint(T(10), 5.0),
             new KpiSeriesPoint(T(35), 6.0),
-            new KpiSeriesPoint(T(60), 7.0), // exactly toUtc → bucket 1
+            new KpiSeriesPoint(T(60), 7.0), // exactly toUtc → must land in last bucket
         };
 
-        var result = KpiSeriesBucketer.Bucket(raw2, T(0), T(60), maxPoints: 2);
+        var result = KpiSeriesBucketer.Bucket(raw, T(0), T(60), maxPoints: 2);
 
         Assert.Equal(2, result.Count);
         // Bucket 1 holds both T(35) and T(60); T(60) is later → wins.
         Assert.Equal(7.0, result[1].Value);
     }
 
+    [Fact]
+    public void Bucket_ThreeBucketWindow_PointAtToUtc_LandsInLastBucket()
+    {
+        // 60-minute window / 3 buckets → 20 min each.
+        // A point exactly at T(60) = toUtc must land in bucket 2 (the last),
+        // not overflow.  Use 4 raw points so downsampling is forced (4 > 3).
+        // T(5) → bucket 0: [0,20); T(25) → bucket 1: [20,40);
+        // T(45) → bucket 2: [40,60]; T(60) → bucket 2 (right edge, later → wins).
+        var raw = new[]
+        {
+            new KpiSeriesPoint(T(5),  1.0),
+            new KpiSeriesPoint(T(25), 2.0),
+            new KpiSeriesPoint(T(45), 3.0),
+            new KpiSeriesPoint(T(60), 99.0), // right edge — must land in last bucket
+        };
+
+        var result = KpiSeriesBucketer.Bucket(raw, T(0), T(60), maxPoints: 3);
+
+        Assert.Equal(3, result.Count);
+        // Bucket 2 holds T(45)=3.0 and T(60)=99.0; T(60) is later → wins.
+        Assert.Equal(99.0, result[2].Value);
+    }
+
     // -----------------------------------------------------------------------
     // Empty buckets omitted — no gap-filling
     // -----------------------------------------------------------------------
diff --git a/tests/ZB.MOM.WW.ScadaBridge.KpiHistory.Tests/KpiHistoryRecorderActorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.KpiHistory.Tests/KpiHistoryRecorderActorTests.cs
index d244e701..80f21421 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.KpiHistory.Tests/KpiHistoryRecorderActorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.KpiHistory.Tests/KpiHistoryRecorderActorTests.cs
@@ -64,6 +64,39 @@ public class KpiHistoryRecorderActorTests : TestKit
             throw new InvalidOperationException("simulated source failure");
     }
 
+    /// <summary>
+    /// A source that throws on the first <see cref="CollectAsync"/> call and returns a
+    /// healthy sample on every subsequent call. Used to drive a faulted first tick followed
+    /// by a healthy second tick on the <em>same</em> actor instance.
+    /// </summary>
+    private sealed class ThrowOnceSource : IKpiSampleSource
+    {
+        private int _callCount;
+
+        public string Source => KpiSources.SiteCallAudit;
+
+        public Task<IReadOnlyList<KpiSample>> CollectAsync(
+            DateTime capturedAtUtc, CancellationToken cancellationToken = default)
+        {
+            if (Interlocked.Increment(ref _callCount) == 1)
+                throw new InvalidOperationException("simulated first-call source failure");
+
+            IReadOnlyList<KpiSample> samples = new[]
+            {
+                new KpiSample
+                {
+                    Source = Source,
+                    Metric = "RecoveredSample",
+                    Scope = KpiScopes.Global,
+                    ScopeKey = null,
+                    Value = 1,
+                    CapturedAtUtc = capturedAtUtc,
+                },
+            };
+            return Task.FromResult(samples);
+        }
+    }
+
     /// <summary>
     /// Recording repository fake. Captures the samples handed to
     /// <see cref="RecordSamplesAsync"/> and the cut-off handed to
@@ -73,13 +106,19 @@ public class KpiHistoryRecorderActorTests : TestKit
     {
         private readonly object _gate = new();
         private readonly List<KpiSample> _recorded = new();
+        private DateTime? _purgeCutoff;
 
         public IReadOnlyList<KpiSample> Recorded
         {
             get { lock (_gate) { return _recorded.ToArray(); } }
         }
 
-        public DateTime? PurgeCutoff { get; private set; }
+        // PurgeOlderThanAsync runs on a threadpool thread; guard the field with
+        // the same _gate lock used by _recorded so test-thread reads are race-free.
+        public DateTime? PurgeCutoff
+        {
+            get { lock (_gate) { return _purgeCutoff; } }
+        }
 
         public Task RecordSamplesAsync(
             IReadOnlyCollection<KpiSample> samples, CancellationToken cancellationToken = default)
@@ -98,7 +137,7 @@ public class KpiHistoryRecorderActorTests : TestKit
 
         public Task<int> PurgeOlderThanAsync(DateTime before, CancellationToken cancellationToken = default)
         {
-            PurgeCutoff = before;
+            lock (_gate) { _purgeCutoff = before; }
             return Task.FromResult(0);
         }
     }
@@ -188,27 +227,28 @@ public class KpiHistoryRecorderActorTests : TestKit
     [Fact]
     public void FaultedTick_DoesNotCrashActor_AndSubsequentTickStillRuns()
     {
+        // ThrowOnceSource throws on the first CollectAsync call and returns a healthy
+        // sample on every subsequent call.  This lets us send two ticks to the SAME
+        // actor instance and verify that:
+        //   • The first tick (faulted source) records nothing but does not crash the actor.
+        //   • The second tick reaches the same actor and records the recovered sample,
+        //     proving the singleton's message loop is still alive after a faulted pass.
         var repository = new RecordingRepository();
-        // A pass containing ONLY a throwing source records nothing but must not crash the
-        // actor; a later healthy tick proves the singleton survived.
-        var sp = BuildServiceProvider(repository, new ThrowingSource());
+        var sp = BuildServiceProvider(repository, new ThrowOnceSource());
         var actor = CreateActor(sp);
 
-        // First tick: the only source throws — caught per-source, nothing written, actor lives.
+        // First tick: source throws on first call — caught per-source, nothing written, actor lives.
         actor.Tell(KpiHistoryRecorderActor.SampleTick.Instance);
         AwaitAssert(
             () => Assert.Empty(repository.Recorded),
-            duration: TimeSpan.FromSeconds(1),
+            duration: TimeSpan.FromSeconds(2),
             interval: TimeSpan.FromMilliseconds(50));
 
-        // Second tick on a fresh actor backed by a healthy source proves the message loop is
-        // still alive and the recorder still records after a faulted pass on the prior actor.
-        var healthyRepo = new RecordingRepository();
-        var healthySp = BuildServiceProvider(healthyRepo, new HealthySource());
-        var healthyActor = CreateActor(healthySp);
-        healthyActor.Tell(KpiHistoryRecorderActor.SampleTick.Instance);
+        // Second tick to the SAME actor: source now returns a healthy sample.
+        // AwaitAssert confirms the actor processed the message and recorded it.
+        actor.Tell(KpiHistoryRecorderActor.SampleTick.Instance);
         AwaitAssert(
-            () => Assert.Equal(2, healthyRepo.Recorded.Count),
+            () => Assert.Single(repository.Recorded),
             duration: TimeSpan.FromSeconds(3),
             interval: TimeSpan.FromMilliseconds(50));
     }