Fix extreme zoom in showing data as missing instead of interpolating the polyline

lib/session/decimator.dart

@@ -3,13 +3,21 @@ import 'dart:collection';
 import '../proto/messages.pb.dart';
 import 'packet_buffer.dart';
 
-/// One column of decimated data. `hasData=false` means no packet contributed
+/// One column of decimated data.
-/// a value for this channel in the column's time slice.
+///
+/// Tri-state: a column is either
+///   - `hasData`  — at least one packet in the slice carried this channel
+///     (renders as a line/min-max span),
+///   - `isGap`    — a packet landed in the slice but had no value for this
+///     channel (renders as hatched missing-data),
+///   - neither    — no packet at all fell in the slice (empty, skipped; the
+///     polyline interpolates across).
 class DecimatedColumn {
-  const DecimatedColumn(this.min, this.max, this.hasData);
+  const DecimatedColumn(this.min, this.max, this.hasData, this.isGap);
   final double min;
   final double max;
   final bool hasData;
+  final bool isGap;
 }
 
 /// Classification of a missing-data segment.
@@ -82,14 +90,18 @@ class Decimator {
     final mins = List<double>.filled(pixelWidth, double.infinity);
     final maxs = List<double>.filled(pixelWidth, double.negativeInfinity);
     final has = List<bool>.filled(pixelWidth, false);
+    final gap = List<bool>.filled(pixelWidth, false);
 
     for (final p in buffer.iterateRange(startUs, endUs)) {
-      final v = _channelValue(p, channel);
-      if (v == null) continue;
       final tsUs = p.timestampUs.toInt();
       var col = ((tsUs - startUs) / pixUs).floor();
       if (col < 0) col = 0;
       if (col >= pixelWidth) col = pixelWidth - 1;
+      final v = _channelValue(p, channel);
+      if (v == null) {
+        gap[col] = true;
+        continue;
+      }
       if (!has[col]) {
         mins[col] = v;
         maxs[col] = v;
@@ -101,7 +113,9 @@ class Decimator {
     }
     return List.generate(
       pixelWidth,
-      (i) => DecimatedColumn(mins[i], maxs[i], has[i]),
+      // A column with at least one value renders as data even if some other
+      // packet in the same slice was missing the field.
+      (i) => DecimatedColumn(mins[i], maxs[i], has[i], gap[i] && !has[i]),
     );
   }
 
@@ -136,7 +150,7 @@ class Decimator {
     final hatched = <({int startPx, int endPx})>[];
     int? gapStart;
     for (var i = 0; i < cols.length; i++) {
-      if (!cols[i].hasData) {
+      if (cols[i].isGap) {
         gapStart ??= i;
       } else if (gapStart != null) {
         hatched.add((startPx: gapStart, endPx: i - 1));

lib/ui/chart_painter.dart

@@ -106,10 +106,14 @@ class ChartPainter extends CustomPainter {
     bool started = false;
     for (var i = 0; i < cols.length; i++) {
       final c = cols[i];
-      if (!c.hasData) {
+      // Real missing-data gap: break the polyline.
+      if (c.isGap) {
         started = false;
         continue;
       }
+      // Empty column (no packet fell here — sub-packet-period zoom). Let the
+      // path interpolate across.
+      if (!c.hasData) continue;
       final x = xForCol(i);
       final yMinPx = yForVal(c.max); // Higher value = smaller y.
       final yMaxPx = yForVal(c.min);