fix(dashboard): downsample history graphs + readable tooltip time

The agent reports every few seconds, so multi-hour history series were hundreds–
thousands of points — a dense, noisy line (esp. CPU). Bucket-average server-side
to ~120 points (keeps the shape, drops the noise) for both the history-graph
widget and the host-detail charts. Also fix the chart tooltip title showing the
raw epoch-ms (e.g. 1,781,720,471,459) — format it as HH:MM.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/host_agent/routes.py

@@ -461,11 +461,35 @@ async def widget_history():
                     key = "disk_root" if p.resource_name != host.name else p.metric_name
                     series[key].append([int(p.recorded_at.timestamp() * 1000), round(p.value, 2)])
 
+    # Downsample: the agent reports every few seconds, so a multi-hour window is
+    # hundreds–thousands of points — a noisy, unreadable line. Bucket-average to a
+    # readable point count, keeping the shape.
+    series = {k: _downsample(v) for k, v in series.items()}
     return await render_template(
         "widget_history.html", host=host, series=series, hours=hours, wid=wid,
     )
 
 
+def _downsample(series: list[list], target: int = 120) -> list[list]:
+    """Bucket-average a time series [[epoch_ms, value], …] down to ~target points.
+
+    Consecutive samples are grouped into equal buckets and averaged (x = bucket
+    midpoint). Cheap, order-preserving, and smooths the high-frequency noise that
+    makes dense agent series hard to read.
+    """
+    n = len(series)
+    if n <= target:
+        return series
+    bucket = (n + target - 1) // target
+    out: list[list] = []
+    for i in range(0, n, bucket):
+        chunk = series[i:i + bucket]
+        mid = chunk[len(chunk) // 2][0]
+        avg = sum(p[1] for p in chunk) / len(chunk)
+        out.append([mid, round(avg, 2)])
+    return out
+
+
 # Host-level metrics charted on the detail page (sub-resources are shown as
 # current-value lists, not time series, to keep the page readable).
 HISTORY_METRICS = (
@@ -520,7 +544,9 @@ async def _history_for_host(session, host_name: str, since) -> dict[str, list]:
     series: dict[str, list] = {m: [] for m in HISTORY_METRICS}
     for p in rows:
         series[p.metric_name].append([int(p.recorded_at.timestamp() * 1000), round(p.value, 2)])
-    return series
+    # Downsample to a readable point count (see _downsample) — raw agent cadence
+    # is too dense to read over a multi-hour window.
+    return {m: _downsample(v) for m, v in series.items()}
 
 
 @host_agent_bp.get("/<host_id>/")

plugins/host_agent/templates/host_detail.html

@@ -216,7 +216,10 @@
       x: { type: "linear", ticks: { callback: (v) => fmtTime(v), maxTicksLimit: 8, color: "#8a8a92", font: { size: 10 } }, grid: { color: "#30303a" } },
       y: { beginAtZero: true, max: ymax, ticks: { color: "#8a8a92", font: { size: 10 } }, grid: { color: "#30303a" } },
     },
-    plugins: { legend: { labels: { color: "#b8b8b0", boxWidth: 10, font: { size: 11 } } } },
+    plugins: {
+      legend: { labels: { color: "#b8b8b0", boxWidth: 10, font: { size: 11 } } },
+      tooltip: { callbacks: { title: function (items) { return items.length ? fmtTime(items[0].parsed.x) : ""; } } },
+    },
   });
   const mk = (id, datasets, ymax) => {
     const el = document.getElementById(id);

plugins/host_agent/templates/widget_history.html

@@ -48,7 +48,10 @@
         // still show the real %, so the values stay honest.
         y: { beginAtZero: true, grace: "8%", ticks: { color: "#8a8a92", font: { size: 10 } }, grid: { color: "#30303a" } },
       },
-      plugins: { legend: { labels: { color: "#b8b8b0", boxWidth: 10, font: { size: 11 } } } },
+      plugins: {
+        legend: { labels: { color: "#b8b8b0", boxWidth: 10, font: { size: 11 } } },
+        tooltip: { callbacks: { title: function (items) { return items.length ? fmtTime(items[0].parsed.x) : ""; } } },
+      },
     },
   });
 })();