diff --git a/src/features/dashboard/helpers.test.ts b/src/features/dashboard/helpers.test.ts
new file mode 100644
index 0000000..0bde2a6
--- /dev/null
+++ b/src/features/dashboard/helpers.test.ts
@@ -0,0 +1,62 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { describe, expect, it } from 'vitest';
+import type { Metric } from './types/metrics';
+import { deltaHistograms } from './helpers';
+
+const total = (timestamp: string, count: number, sum: number, nodeId?: number): Metric => ({
+  '@type': 'Histogram',
+  metric: 'delivery.attempt-time',
+  count,
+  sum,
+  timestamp,
+  nodeId,
+});
+
+const sums = (samples: Metric[]) =>
+  samples.reduce(
+    (acc, m) => (m['@type'] === 'Histogram' ? { count: acc.count + m.count, sum: acc.sum + m.sum } : acc),
+    { count: 0, sum: 0 },
+  );
+
+describe('deltaHistograms', () => {
+  it('diffs one node’s running totals', () => {
+    const out = deltaHistograms([
+      total('2026-09-30T10:00:00Z', 10, 1000),
+      total('2026-09-30T11:00:00Z', 12, 1400),
+      total('2026-09-30T12:00:00Z', 15, 2000),
+    ]);
+    expect(sums(out)).toEqual({ count: 5, sum: 1000 });
+  });
+
+  it('diffs each node against itself, never against another node', () => {
+    // A busy fast node and a quiet node with one slow attempt, interleaved
+    const out = deltaHistograms([
+      total('2026-09-30T10:00:00Z', 100, 20_000, 0),
+      total('2026-09-30T10:00:01Z', 4, 300_000, 1),
+      total('2026-09-30T11:00:00Z', 110, 22_000, 0),
+      total('2026-09-30T11:00:01Z', 4, 300_000, 1),
+      total('2026-09-30T12:00:00Z', 120, 24_000, 0),
+      total('2026-09-30T12:00:01Z', 5, 301_000, 1),
+    ]);
+    expect(sums(out)).toEqual({ count: 21, sum: 5_000 });
+  });
+
+  it('skips a restart, where a node’s totals start again from zero', () => {
+    const out = deltaHistograms([
+      total('2026-09-30T10:00:00Z', 50, 5000, 2),
+      total('2026-09-30T11:00:00Z', 3, 300, 2),
+      total('2026-09-30T12:00:00Z', 5, 700, 2),
+    ]);
+    expect(sums(out)).toEqual({ count: 2, sum: 400 });
+  });
+
+  it('passes counters and gauges through untouched', () => {
+    const counter: Metric = { '@type': 'Counter', metric: 'auth.success', count: 3, nodeId: 1 };
+    expect(deltaHistograms([counter])).toEqual([counter]);
+  });
+});
diff --git a/src/features/dashboard/helpers.ts b/src/features/dashboard/helpers.ts
index 7e45560..a410c9a 100644
--- a/src/features/dashboard/helpers.ts
+++ b/src/features/dashboard/helpers.ts
@@ -1,7 +1,10 @@
 /*
  * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <hello@stalw.art>
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
  *
  * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
+ *
+ * Modified by Coffey Labs in 2026 for INBUXA.
  */
 
 import type { Metric, MetricId, Period } from './types/metrics';
@@ -15,19 +18,23 @@ export function metricToScalar(m: Metric): number {
   return m.count;
 }
 
+// INBUXA: histograms are running totals per node, so they're diffed per
+// node; in a cluster, diffing across nodes subtracts one node's total from
+// another's.
 export function deltaHistograms(samples: Metric[]): Metric[] {
   const nonHistograms: Metric[] = [];
-  const byMetric = new Map<MetricId, Metric[]>();
+  const byMetric = new Map<string, Metric[]>();
 
   for (const m of samples) {
     if (m['@type'] !== 'Histogram') {
       nonHistograms.push(m);
       continue;
     }
-    let list = byMetric.get(m.metric);
+    const key = `${m.metric}|${m.nodeId ?? ''}`;
+    let list = byMetric.get(key);
     if (!list) {
       list = [];
-      byMetric.set(m.metric, list);
+      byMetric.set(key, list);
     }
     list.push(m);
   }
diff --git a/src/features/dashboard/types/metrics.ts b/src/features/dashboard/types/metrics.ts
index 1c6600c..0495a80 100644
--- a/src/features/dashboard/types/metrics.ts
+++ b/src/features/dashboard/types/metrics.ts
@@ -1,15 +1,19 @@
 /*
  * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <hello@stalw.art>
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
  *
  * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
+ *
+ * Modified by Coffey Labs in 2026 for INBUXA.
  */
 
 export type MetricId = string;
 
+// INBUXA: nodeId is the node that wrote the sample (servers from 2026.9.30 on)
 export type Metric =
-  | { '@type': 'Counter'; metric: MetricId; count: number; timestamp?: string }
-  | { '@type': 'Gauge'; metric: MetricId; count: number; timestamp?: string }
-  | { '@type': 'Histogram'; metric: MetricId; count: number; sum: number; timestamp?: string };
+  | { '@type': 'Counter'; metric: MetricId; count: number; timestamp?: string; nodeId?: number }
+  | { '@type': 'Gauge'; metric: MetricId; count: number; timestamp?: string; nodeId?: number }
+  | { '@type': 'Histogram'; metric: MetricId; count: number; sum: number; timestamp?: string; nodeId?: number };
 
 export type Period = { kind: 'preset'; preset: PresetKey } | { kind: 'custom'; from: Date; to: Date };