fix: preserve chart history across reconnects, show gaps for missing data

Only wipe real (non-RTT) plugin data on an actual client reboot (boot
flag), not on every ordinary OVERDUE/DOWN -> UP recovery. A transient
network blip no longer erases CPU/memory/etc. history.

Also split the shared time-series chart into separate line/area segments
wherever the gap between samples is much larger than the typical spacing,
so missing data (host overdue, or history that simply hasn't accumulated
across a drop) renders as a visual gap instead of an interpolated line.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DEimzMv4Q5EjFg3hoiZ69T

hbd/server/templates/plugins.html

@@ -847,16 +847,38 @@
         const yRange = yHigh - yLow || 1;
         const y = v => PAD.top + cH - ((v - yLow) / yRange) * cH;
 
-        // Build polyline points and filled area path
-        const linePoints = pts.map(p => `${x(p.t).toFixed(1)},${y(p.v).toFixed(1)}`).join(' ');
-        const areaPath = `M${x(pts[0].t).toFixed(1)},${(PAD.top + cH).toFixed(1)} ` +
-          pts.map(p => `L${x(p.t).toFixed(1)},${y(p.v).toFixed(1)}`).join(' ') +
-          ` L${x(pts[pts.length-1].t).toFixed(1)},${(PAD.top + cH).toFixed(1)} Z`;
-
         // Color based on latest value
         const latest = pts[pts.length - 1].v;
         const { stroke: strokeColor, fill: fillColor } = opts.colorFor(latest);
 
+        // Split into segments wherever the gap between consecutive samples is
+        // much larger than the typical spacing (e.g. the host was overdue/down
+        // for a while) — draw each segment separately so missing data reads as
+        // a visual gap instead of an interpolated line across dead time.
+        const deltas = [];
+        for (let i = 1; i < pts.length; i++) deltas.push(pts[i].t - pts[i - 1].t);
+        deltas.sort((a, b) => a - b);
+        const medianDelta = deltas[Math.floor(deltas.length / 2)];
+        const gapThreshold = medianDelta * 2.5;
+
+        const segments = [[pts[0]]];
+        for (let i = 1; i < pts.length; i++) {
+          if (pts[i].t - pts[i - 1].t > gapThreshold) segments.push([]);
+          segments[segments.length - 1].push(pts[i]);
+        }
+
+        let linePolylines = '';
+        let areaPaths = '';
+        for (const seg of segments) {
+          if (seg.length < 2) continue;
+          const segPoints = seg.map(p => `${x(p.t).toFixed(1)},${y(p.v).toFixed(1)}`).join(' ');
+          linePolylines += `<polyline points="${segPoints}" fill="none" stroke="${strokeColor}" stroke-width="1.5" stroke-linejoin="round"/>`;
+          const segArea = `M${x(seg[0].t).toFixed(1)},${(PAD.top + cH).toFixed(1)} ` +
+            seg.map(p => `L${x(p.t).toFixed(1)},${y(p.v).toFixed(1)}`).join(' ') +
+            ` L${x(seg[seg.length-1].t).toFixed(1)},${(PAD.top + cH).toFixed(1)} Z`;
+          areaPaths += `<path d="${segArea}" fill="${fillColor}" opacity="0.6"/>`;
+        }
+
         // Compute nice tick step for ~3-5 grid lines
         const rawStep = yRange / 4;
         const mag = Math.pow(10, Math.floor(Math.log10(rawStep || 1)));
@@ -890,8 +912,8 @@
           <line x1="${PAD.left}" y1="${PAD.top}" x2="${PAD.left}" y2="${PAD.top + cH}" stroke="#ccc" stroke-width="1"/>
           <line x1="${PAD.left}" y1="${PAD.top + cH}" x2="${PAD.left + cW}" y2="${PAD.top + cH}" stroke="#ccc" stroke-width="1"/>
           <g clip-path="url(#${opts.clipId})">
-            <path d="${areaPath}" fill="${fillColor}" opacity="0.6"/>
-            <polyline points="${linePoints}" fill="none" stroke="${strokeColor}" stroke-width="1.5" stroke-linejoin="round"/>
+            ${areaPaths}
+            ${linePolylines}
           </g>
           ${xLabels}
         </svg>`;

hbd/server/udp.py

@@ -521,13 +521,17 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         # Transition to UP and log/notify if appropriate
         lasts = conn.state
         d = conn.newstate(hbdcls.Connection.UP, now)
-        # On reboot, pre-boot plugin data and derived alerts are stale.
-        # Cancel all plugin timers and wipe plugin state so timers restart
-        # cleanly from the first two post-boot samples.
-        for pname in list(host.plugin_timers):
-            host.cancel_plugin_timer(pname)
-        for pname in [k for k in host.plugin_data if not _is_rtt_key(k)]:
-            del host.plugin_data[pname]
+        if boot:
+            # On reboot, pre-boot plugin data and derived alerts are stale.
+            # Cancel all plugin timers and wipe plugin state so timers restart
+            # cleanly from the first two post-boot samples. An ordinary
+            # reconnect (no boot flag) doesn't invalidate the client's
+            # already-collected data, so it's left alone — this keeps chart
+            # history intact across a transient network blip.
+            for pname in list(host.plugin_timers):
+                host.cancel_plugin_timer(pname)
+            for pname in [k for k in host.plugin_data if not _is_rtt_key(k)]:
+                del host.plugin_data[pname]
         stale_plugin_keys = [
             k for k in host.alert_states
             if k not in ("rtt",) and not k.startswith("connectivity.")