feat: replay chart with zoomable viewpor

src/pygui/ISC/Tabs/WaferMapTab.qml

@@ -182,6 +182,26 @@ Item {
                     }
                 }
 
+                Rectangle {
+                    id: replayChartPane
+                    Layout.fillWidth: true
+                    Layout.fillHeight: false
+                    Layout.preferredHeight: 260
+                    Layout.minimumHeight: 180
+                    visible: streamController.mode !== "live" && streamController.loadedFile !== ""
+                    color: Theme.cardBackground
+                    border.color: Theme.cardBorder
+                    border.width: 1
+                    radius: Theme.radiusMd
+
+                    ReplayChart {
+                        anchors.fill: parent
+                        anchors.margins: 8
+                        sensorNames: streamController.graphSensorNames ? streamController.graphSensorNames.split(",") : []
+                        seriesData: JSON.parse(streamController.graphSeriesJson || "[]")
+                    }
+                }
+
                 Item {
                     Layout.fillWidth: true
                     visible: transportBar.hasContent

src/pygui/ISC/Tabs/components/ReplayChart.qml

@@ -0,0 +1,136 @@
+import QtQuick
+import QtQuick.Controls
+import QtQuick.Layouts
+import ISC
+import ISC.Wafer
+
+// ===== Replay Chart =====
+// Zoomable whole-run multi-sensor chart (C# parity: PopupChartForm.cs).
+// Embedded inline in WaferMapTab's replay slot rather than a separate popup
+// window. Mouse wheel zooms around the cursor, drag pans; the strip below
+// shows the aggregate min/max/avg over the *visible* range (not the whole
+// run), matching PopupChartForm's recalc_stats. Cursor/playhead tracking is
+// out of scope — see specs/plans/2026-07-10-popup-chart.md and
+// docs/adr/0004-graphquickitem-viewport-for-replay-chart.md.
+Item {
+    id: root
+
+    property alias sensorNames: chart.sensorNames
+    property alias seriesData: chart.seriesData
+
+    ColumnLayout {
+        anchors.fill: parent
+        spacing: 8
+
+        Item {
+            Layout.fillWidth: true
+            Layout.fillHeight: true
+
+            GraphQuickItem {
+                id: chart
+                anchors.fill: parent
+                showMinMaxMarkers: true
+                title: "Replay Chart"
+                xLabel: "Measurement Interval"
+                yLabel: "Temperature (°C)"
+            }
+
+            MouseArea {
+                id: interaction
+                anchors.fill: parent
+                acceptedButtons: Qt.LeftButton
+                property real lastX: 0
+
+                onWheel: function(wheel) {
+                    var frac = Math.max(0, Math.min(1, wheel.x / width));
+                    var factor = wheel.angleDelta.y > 0 ? 0.8 : 1.25;
+                    chart.zoomAtFraction(frac, factor);
+                    wheel.accepted = true;
+                }
+                onPressed: function(mouse) {
+                    lastX = mouse.x;
+                }
+                onPositionChanged: function(mouse) {
+                    if (pressed && width > 0) {
+                        chart.panByFraction(-(mouse.x - lastX) / width);
+                        lastX = mouse.x;
+                    }
+                }
+            }
+        }
+
+        // ── Min/Max/Avg readout strip + Reset Zoom ─────────────────────
+        RowLayout {
+            Layout.fillWidth: true
+            spacing: 16
+
+            Label {
+                text: "Min:"
+                color: Theme.sensorLow
+                font.family: Theme.uiFontFamily
+                font.pixelSize: Theme.fontSm
+                font.weight: Font.Medium
+            }
+            Label {
+                text: chart.viewMin.toFixed(2) + (chart.viewMinSensor ? "  (" + chart.viewMinSensor + ")" : "")
+                color: Theme.headingColor
+                font.family: Theme.codeFontFamily
+                font.pixelSize: Theme.fontSm
+            }
+
+            Label {
+                text: "Max:"
+                color: Theme.sensorHigh
+                font.family: Theme.uiFontFamily
+                font.pixelSize: Theme.fontSm
+                font.weight: Font.Medium
+            }
+            Label {
+                text: chart.viewMax.toFixed(2) + (chart.viewMaxSensor ? "  (" + chart.viewMaxSensor + ")" : "")
+                color: Theme.headingColor
+                font.family: Theme.codeFontFamily
+                font.pixelSize: Theme.fontSm
+            }
+
+            Label {
+                text: "Avg:"
+                color: Theme.sideMutedText
+                font.family: Theme.uiFontFamily
+                font.pixelSize: Theme.fontSm
+                font.weight: Font.Medium
+            }
+            Label {
+                text: chart.viewAvg.toFixed(2)
+                color: Theme.headingColor
+                font.family: Theme.codeFontFamily
+                font.pixelSize: Theme.fontSm
+            }
+
+            Item { Layout.fillWidth: true }
+
+            Button {
+                id: resetZoomBtn
+                implicitHeight: 32
+                hoverEnabled: true
+                text: "Reset Zoom"
+                font.family: Theme.uiFontFamily
+                font.pixelSize: Theme.fontSm
+                font.weight: Font.Medium
+                background: Rectangle {
+                    radius: Theme.radiusSm
+                    color: resetZoomBtn.pressed ? Theme.transportButtonHover : Theme.transportButtonBg
+                    border.color: Theme.sideBorder
+                    border.width: 1
+                }
+                contentItem: Text {
+                    text: resetZoomBtn.text
+                    color: Theme.headingColor
+                    font: resetZoomBtn.font
+                    horizontalAlignment: Text.AlignHCenter
+                    verticalAlignment: Text.AlignVCenter
+                }
+                onClicked: chart.resetZoom()
+            }
+        }
+    }
+}

src/pygui/ISC/Tabs/components/qmldir

@@ -23,3 +23,4 @@ EditableScrubStat 1.0 EditableScrubStat.qml
 ComparisonChartCard 1.0 ComparisonChartCard.qml
 OverlapMapCard 1.0 OverlapMapCard.qml
 ComparisonSidePanel 1.0 ComparisonSidePanel.qml
+ReplayChart 1.0 ReplayChart.qml

src/pygui/backend/visualization/graph_quick_item.py

@@ -21,7 +21,9 @@ Usage from QML::
 
 from __future__ import annotations
 
-from PySide6.QtCore import Property, QRectF, Qt, Signal
+import math
+
+from PySide6.QtCore import Property, QRectF, Qt, Signal, Slot
 from PySide6.QtGui import QColor, QFont, QFontMetrics, QPainter, QPen
 from PySide6.QtQml import QmlElement
 from PySide6.QtQuick import QQuickPaintedItem
@@ -32,6 +34,47 @@ QML_IMPORT_NAME = "ISC.Wafer"
 QML_IMPORT_MAJOR_VERSION = 1
 
 
+def viewport_stats(
+    series: list[list[float]],
+    names: list[str],
+    start: int,
+    end: int,
+) -> tuple[float, float, float, str, str]:
+    """Aggregate min/max/avg across every sensor's points in `[start, end]`.
+
+    Mirrors PopupChartForm.cs's `recalc_stats`: one running min/max/avg over
+    every visible point from every series, plus which sensor achieved each
+    extreme. Non-numeric values are skipped. No points in range -> zeros and
+    empty sensor names.
+    """
+    vmin = math.inf
+    vmax = -math.inf
+    total = 0.0
+    count = 0
+    min_sensor = ""
+    max_sensor = ""
+    for i, s in enumerate(series):
+        name = names[i] if i < len(names) else ""
+        lo = max(0, start)
+        hi = min(len(s) - 1, end)
+        for j in range(lo, hi + 1):
+            try:
+                v = float(s[j])
+            except (TypeError, ValueError):
+                continue
+            if v < vmin:
+                vmin = v
+                min_sensor = name
+            if v > vmax:
+                vmax = v
+                max_sensor = name
+            total += v
+            count += 1
+    if count == 0:
+        return (0.0, 0.0, 0.0, "", "")
+    return (vmin, vmax, total / count, min_sensor, max_sensor)
+
+
 @QmlElement
 class GraphQuickItem(QQuickPaintedItem):
     """Painted line chart; driven by series data passed via QML property bindings."""
@@ -42,6 +85,8 @@ class GraphQuickItem(QQuickPaintedItem):
     xLabelChanged = Signal()
     yLabelChanged = Signal()
     colorsChanged = Signal()
+    viewportChanged = Signal()
+    viewStatsChanged = Signal()
 
     def __init__(self, parent=None) -> None:
         super().__init__(parent)
@@ -74,6 +119,18 @@ class GraphQuickItem(QQuickPaintedItem):
         self._max_y: float = 150.0
         self._padding: dict[str, int] = {"left": 60, "right": 20, "top": 30, "bottom": 50}
 
+        # Viewport (replay chart zoom/pan): defaults reproduce "whole series",
+        # so the Graph tab (which never sets these) is unaffected. See
+        # docs/adr/0004-graphquickitem-viewport-for-replay-chart.md.
+        self._view_start: int = 0
+        self._view_end: int = -1
+        self._show_min_max_markers: bool = False
+        self._view_min: float = 0.0
+        self._view_max: float = 0.0
+        self._view_avg: float = 0.0
+        self._view_min_sensor: str = ""
+        self._view_max_sensor: str = ""
+
     # ── Qt properties (QML-bindable) ──────────────────────────────────────────
 
     @Property("QVariantList", notify=seriesDataChanged)
@@ -83,7 +140,16 @@ class GraphQuickItem(QQuickPaintedItem):
     @seriesData.setter  # type: ignore[no-redef]
     def seriesData(self, val: list) -> None:
         self._series_data = [list(s) for s in (val or [])]
+        # New data resets the viewport to the full range -- matches
+        # PopupChartForm.cs's SetData(), which resets AxisX.Minimum/Maximum
+        # before RecalculateAxesScale(). Without this, a stale viewport left
+        # over from a previous (longer) series can point entirely past the
+        # new data -- every per-series slice becomes empty and the chart
+        # silently renders nothing instead of the new data.
+        self._view_start = 0
+        self._view_end = -1
         self._auto_range()
+        self._recompute_view_stats()
         self.seriesDataChanged.emit()
         self.update()
 
@@ -94,9 +160,130 @@ class GraphQuickItem(QQuickPaintedItem):
     @sensorNames.setter  # type: ignore[no-redef]
     def sensorNames(self, val: list) -> None:
         self._sensor_names = list(val or [])
+        self._recompute_view_stats()
         self.sensorNamesChanged.emit()
         self.update()
 
+    # ── Viewport properties (replay chart zoom/pan) ────────────────────────
+
+    @Property(int, notify=viewportChanged)
+    def viewStartIndex(self) -> int:
+        return self._view_start
+
+    @viewStartIndex.setter  # type: ignore[no-redef]
+    def viewStartIndex(self, val: int) -> None:
+        v = max(0, int(val))
+        if v == self._view_start:
+            return
+        self._view_start = v
+        self._on_viewport_changed()
+
+    @Property(int, notify=viewportChanged)
+    def viewEndIndex(self) -> int:
+        return self._view_end
+
+    @viewEndIndex.setter  # type: ignore[no-redef]
+    def viewEndIndex(self, val: int) -> None:
+        v = int(val)
+        if v == self._view_end:
+            return
+        self._view_end = v
+        self._on_viewport_changed()
+
+    @Property(bool, notify=viewportChanged)
+    def showMinMaxMarkers(self) -> bool:
+        return self._show_min_max_markers
+
+    @showMinMaxMarkers.setter  # type: ignore[no-redef]
+    def showMinMaxMarkers(self, val: bool) -> None:
+        v = bool(val)
+        if v == self._show_min_max_markers:
+            return
+        self._show_min_max_markers = v
+        self.viewportChanged.emit()
+        self.update()
+
+    @Property(float, notify=viewStatsChanged)
+    def viewMin(self) -> float:
+        return self._view_min
+
+    @Property(float, notify=viewStatsChanged)
+    def viewMax(self) -> float:
+        return self._view_max
+
+    @Property(float, notify=viewStatsChanged)
+    def viewAvg(self) -> float:
+        return self._view_avg
+
+    @Property(str, notify=viewStatsChanged)
+    def viewMinSensor(self) -> str:
+        return self._view_min_sensor
+
+    @Property(str, notify=viewStatsChanged)
+    def viewMaxSensor(self) -> str:
+        return self._view_max_sensor
+
+    # ── Zoom/pan slots (replay chart interaction; QML MouseArea calls these
+    #    with fractions derived from wheel/drag pixel deltas) ───────────────
+
+    @Slot(float, float)
+    def zoomAtFraction(self, frac: float, factor: float) -> None:
+        """Zoom the viewport around a fractional X position within it.
+
+        `frac` in [0, 1] is where within the current viewport to zoom around;
+        `factor < 1` narrows the window (zoom in), `factor > 1` widens it
+        (zoom out). Clamped to data bounds and a minimum 1-index window.
+        """
+        max_len = max((len(s) for s in self._series_data), default=0)
+        if max_len < 2:
+            return
+        start = float(self._view_start)
+        end = float(self._resolved_view_end())
+        width = max(1.0, end - start)
+        f = max(0.0, min(1.0, frac))
+        center = start + f * width
+        new_width = max(1.0, min(float(max_len - 1), width * max(0.01, factor)))
+        new_start = center - f * new_width
+        new_end = new_start + new_width
+        self._set_viewport_clamped(new_start, new_end, max_len)
+
+    @Slot(float)
+    def panByFraction(self, delta_frac: float) -> None:
+        """Shift the viewport by `delta_frac * window_width` indices."""
+        max_len = max((len(s) for s in self._series_data), default=0)
+        if max_len < 2:
+            return
+        start = float(self._view_start)
+        end = float(self._resolved_view_end())
+        width = max(1.0, end - start)
+        shift = delta_frac * width
+        self._set_viewport_clamped(start + shift, end + shift, max_len)
+
+    @Slot()
+    def resetZoom(self) -> None:
+        """Restore the full-range viewport sentinel (0, -1)."""
+        self._set_viewport(0, -1)
+
+    def _set_viewport_clamped(self, new_start: float, new_end: float, max_len: int) -> None:
+        if new_start < 0.0:
+            new_end -= new_start
+            new_start = 0.0
+        if new_end > max_len - 1:
+            new_start -= new_end - (max_len - 1)
+            new_end = float(max_len - 1)
+        new_start = max(0.0, new_start)
+        self._set_viewport(int(round(new_start)), int(round(new_end)))
+
+    def _set_viewport(self, start: int, end: int) -> None:
+        """Set the viewport bounds directly (not through the QML-facing
+        property setters, so this stays a plain attribute write for mypy)."""
+        new_start = max(0, start)
+        if new_start == self._view_start and end == self._view_end:
+            return
+        self._view_start = new_start
+        self._view_end = end
+        self._on_viewport_changed()
+
     @Property(str, notify=titleChanged)
     def title(self) -> str:
         return self._title
@@ -194,13 +381,38 @@ class GraphQuickItem(QQuickPaintedItem):
 
     # ── internal ──────────────────────────────────────────────────────────────
 
+    def _resolved_view_end(self) -> int:
+        """`viewEndIndex` with the -1 ("last index") sentinel resolved."""
+        max_len = max((len(s) for s in self._series_data), default=0)
+        if self._view_end < 0 or self._view_end > max_len - 1:
+            return max_len - 1
+        return self._view_end
+
+    def _on_viewport_changed(self) -> None:
+        self._auto_range()
+        self._recompute_view_stats()
+        self.viewportChanged.emit()
+        self.update()
+
+    def _recompute_view_stats(self) -> None:
+        end = self._resolved_view_end()
+        vmin, vmax, avg, min_sensor, max_sensor = viewport_stats(
+            self._series_data, self._sensor_names, self._view_start, end
+        )
+        self._view_min, self._view_max, self._view_avg = vmin, vmax, avg
+        self._view_min_sensor, self._view_max_sensor = min_sensor, max_sensor
+        self.viewStatsChanged.emit()
+
     def _auto_range(self) -> None:
-        """Set Y range to cover all data with 10% padding."""
+        """Set Y range to cover the current viewport's data with 10% padding."""
+        start = max(0, self._view_start)
+        end = self._resolved_view_end()
         all_vals: list[float] = []
         for series in self._series_data:
-            for v in series:
+            hi = min(len(series) - 1, end)
+            for i in range(start, hi + 1):
                 try:
-                    all_vals.append(float(v))
+                    all_vals.append(float(series[i]))
                 except (ValueError, TypeError):
                     pass
         if not all_vals:
@@ -319,18 +531,23 @@ class GraphQuickItem(QQuickPaintedItem):
                 self._x_label,
             )
 
-        # --- X-axis ticks ---
+        # --- Viewport slice (replay chart zoom/pan; full range by default,
-        num_points = max(len(s) for s in self._series_data) if self._series_data else 0
+        #     so unset viewport reproduces the pre-viewport rendering) ---
-        if num_points > 1:
+        view_start = max(0, self._view_start)
-            n_x_ticks = min(num_points, max(2, int(plot_w // 80)))
+        view_end = self._resolved_view_end()
-            x_tick_step = (num_points - 1) / max(1, n_x_ticks - 1)
+        view_len = max(1, view_end - view_start + 1)
+
+        # --- X-axis ticks (labelled in original, non-sliced index space) ---
+        if view_len > 1:
+            n_x_ticks = min(view_len, max(2, int(plot_w // 80)))
+            x_tick_step = (view_len - 1) / max(1, n_x_ticks - 1)
             for i in range(n_x_ticks):
-                idx = int(round(i * x_tick_step))
+                rel_idx = int(round(i * x_tick_step))
-                x_px = index_to_pixel(idx, num_points, plot_left, plot_w)
+                x_px = index_to_pixel(rel_idx, view_len, plot_left, plot_w)
                 painter.setPen(axis_pen)
                 painter.drawText(
                     int(x_px - 12), int(plot_top + plot_h + 14),
-                    str(idx + 1),
+                    str(view_start + rel_idx + 1),
                 )
 
         # --- Axes boundary ---
@@ -339,28 +556,36 @@ class GraphQuickItem(QQuickPaintedItem):
         painter.drawLine(int(plot_left), int(plot_top), int(plot_left), int(plot_top + plot_h))
         painter.drawLine(int(plot_left), int(plot_top + plot_h), int(plot_left + plot_w), int(plot_top + plot_h))
 
-        # --- Figure out max series length ---
+        # --- Draw each series (sliced to the current viewport) ---
-        max_len = max(len(s) for s in self._series_data) if self._series_data else 0
+        min_pt: tuple[float, float] | None = None
-        if max_len < 1:
+        max_pt: tuple[float, float] | None = None
-            return
+        min_seen = math.inf
-
+        max_seen = -math.inf
-        # --- Draw each series ---
         for si, series in enumerate(self._series_data):
-            if len(series) < 1:
+            hi = min(len(series) - 1, view_end)
+            sliced = series[view_start:hi + 1] if view_start <= hi else []
+            if not sliced:
                 continue
             color = self._series_colors[si % len(self._series_colors)]
             pen = QPen(color, 2)
             painter.setPen(pen)
 
             pts: list[tuple[float, float]] = []
-            for i, val in enumerate(series):
+            for i, val in enumerate(sliced):
                 try:
                     v = float(val)
                 except (ValueError, TypeError):
                     continue
-                x_px = index_to_pixel(i, max_len, plot_left, plot_w)
+                x_px = index_to_pixel(i, view_len, plot_left, plot_w)
                 y_px = value_to_pixel(v, self._min_y, self._max_y, plot_top, plot_h)
                 pts.append((x_px, y_px))
+                if self._show_min_max_markers:
+                    if v < min_seen:
+                        min_seen = v
+                        min_pt = (x_px, y_px)
+                    if v > max_seen:
+                        max_seen = v
+                        max_pt = (x_px, y_px)
 
             if len(pts) < 2:
                 if len(pts) == 1:
@@ -373,6 +598,19 @@ class GraphQuickItem(QQuickPaintedItem):
                     int(pts[i + 1][0]), int(pts[i + 1][1]),
                 )
 
+        # --- Min/max markers (replay chart only; PopupChartForm.cs parity) ---
+        if self._show_min_max_markers:
+            marker_font = QFont()
+            marker_font.setPixelSize(14)
+            marker_font.setBold(True)
+            painter.setFont(marker_font)
+            if max_pt is not None:
+                painter.setPen(QPen(QColor("#FF5757")))
+                painter.drawText(int(max_pt[0] - 6), int(max_pt[1] - 8), "▼")
+            if min_pt is not None:
+                painter.setPen(QPen(QColor("#42A5F5")))
+                painter.drawText(int(min_pt[0] - 6), int(min_pt[1] + 18), "▲")
+
         # --- Legend ---
         if self._show_legend and self._sensor_names:
             legend_font = QFont()

tests/test_graph_quick_item.py

@@ -59,3 +59,231 @@ def test_non_numeric_values_are_skipped_not_crashing():
 
     assert item._min_y == pytest.approx(149.9)
     assert item._max_y == pytest.approx(151.1)
+
+
+# ---- viewport_stats (replay chart min/max/avg readouts) ----
+# Independently hand-computed against PopupChartForm.cs's recalc_stats
+# semantics: aggregate across every sensor's points in [start, end], plus
+# which sensor achieved each extreme.
+
+
+def test_viewport_stats_full_range_aggregate():
+    from pygui.backend.visualization.graph_quick_item import viewport_stats
+
+    series = [[150.0, 151.0, 152.0], [148.0, 147.5, 147.0]]
+    names = ["Sensor 1", "Sensor 2"]
+
+    vmin, vmax, avg, min_sensor, max_sensor = viewport_stats(series, names, 0, 2)
+
+    assert vmin == pytest.approx(147.0)
+    assert vmax == pytest.approx(152.0)
+    assert avg == pytest.approx(149.25)
+    assert min_sensor == "Sensor 2"
+    assert max_sensor == "Sensor 1"
+
+
+def test_viewport_stats_restricted_to_subrange():
+    from pygui.backend.visualization.graph_quick_item import viewport_stats
+
+    series = [[150.0, 151.0, 152.0], [148.0, 147.5, 147.0]]
+    names = ["Sensor 1", "Sensor 2"]
+
+    vmin, vmax, avg, min_sensor, max_sensor = viewport_stats(series, names, 0, 1)
+
+    assert vmin == pytest.approx(147.5)
+    assert vmax == pytest.approx(151.0)
+    assert avg == pytest.approx(149.125)
+    assert min_sensor == "Sensor 2"
+    assert max_sensor == "Sensor 1"
+
+
+def test_viewport_stats_empty_series_returns_zeros():
+    from pygui.backend.visualization.graph_quick_item import viewport_stats
+
+    assert viewport_stats([], [], 0, -1) == (0.0, 0.0, 0.0, "", "")
+
+
+def test_viewport_stats_skips_non_numeric():
+    from pygui.backend.visualization.graph_quick_item import viewport_stats
+
+    series = [[150.0, "bad", 151.0]]
+    names = ["Sensor 1"]
+
+    vmin, vmax, avg, min_sensor, max_sensor = viewport_stats(series, names, 0, 2)
+
+    assert vmin == pytest.approx(150.0)
+    assert vmax == pytest.approx(151.0)
+    assert avg == pytest.approx(150.5)
+    assert min_sensor == "Sensor 1"
+    assert max_sensor == "Sensor 1"
+
+
+# ---- GraphQuickItem viewport properties (replay chart) ----
+# Default viewport (0, -1) means "whole series" so the already-shipped Graph
+# tab (which never sets these) is unaffected — see docs/adr/0004.
+
+
+def test_viewport_defaults_to_whole_range_and_markers_off():
+    from pygui.backend.visualization.graph_quick_item import GraphQuickItem
+
+    item = GraphQuickItem()
+
+    assert item.viewStartIndex == 0
+    assert item.viewEndIndex == -1
+    assert item.showMinMaxMarkers is False
+
+
+def test_view_stats_reflect_default_full_range_viewport():
+    from pygui.backend.visualization.graph_quick_item import GraphQuickItem
+
+    item = GraphQuickItem()
+    item.sensorNames = ["Sensor 1", "Sensor 2"]
+    item.seriesData = [[150.0, 151.0, 152.0], [148.0, 147.5, 147.0]]
+
+    assert item.viewMin == pytest.approx(147.0)
+    assert item.viewMax == pytest.approx(152.0)
+    assert item.viewAvg == pytest.approx(149.25)
+    assert item.viewMinSensor == "Sensor 2"
+    assert item.viewMaxSensor == "Sensor 1"
+
+
+def test_view_stats_restrict_to_narrowed_viewport():
+    from pygui.backend.visualization.graph_quick_item import GraphQuickItem
+
+    item = GraphQuickItem()
+    item.sensorNames = ["Sensor 1", "Sensor 2"]
+    item.seriesData = [[150.0, 151.0, 152.0], [148.0, 147.5, 147.0]]
+
+    item.viewStartIndex = 0
+    item.viewEndIndex = 1
+
+    assert item.viewMin == pytest.approx(147.5)
+    assert item.viewMax == pytest.approx(151.0)
+    assert item.viewAvg == pytest.approx(149.125)
+
+
+def test_auto_range_y_restricted_to_narrowed_viewport():
+    from pygui.backend.visualization.graph_quick_item import GraphQuickItem
+
+    item = GraphQuickItem()
+    item.seriesData = [[150.0, 100.0], [200.0, 120.0]]
+    # Full-range Y (pre-existing behavior): min=100, max=200 -> [90, 210]
+    assert item._min_y == pytest.approx(90.0)
+    assert item._max_y == pytest.approx(210.0)
+
+    # Narrow the viewport to index 0 only: min=150, max=200, range=50, 10% pad
+    item.viewStartIndex = 0
+    item.viewEndIndex = 0
+
+    assert item._min_y == pytest.approx(145.0)
+    assert item._max_y == pytest.approx(205.0)
+
+
+# ---- zoomAtFraction / panByFraction / resetZoom (replay chart interaction) ----
+# 11-point series (indices 0..10) chosen so the zoom/pan arithmetic lands on
+# whole numbers -- avoids rounding ambiguity in the independent hand trace.
+
+
+def _eleven_point_item():
+    from pygui.backend.visualization.graph_quick_item import GraphQuickItem
+
+    item = GraphQuickItem()
+    item.seriesData = [[float(i) for i in range(11)]]
+    return item
+
+
+def test_zoom_in_centers_on_fraction():
+    item = _eleven_point_item()
+
+    # Full range is (0, 10), width 10. Zooming to 40% width centered at the
+    # midpoint (frac=0.5): center=5, new_width=4 -> new_start=3, new_end=7.
+    item.zoomAtFraction(0.5, 0.4)
+
+    assert item.viewStartIndex == 3
+    assert item.viewEndIndex == 7
+
+
+def test_zoom_out_clamps_to_data_bounds():
+    item = _eleven_point_item()
+    item.viewStartIndex = 3
+    item.viewEndIndex = 7
+
+    # width=4, zoom out 3x centered at midpoint (frac=0.5): requested
+    # new_width=12 exceeds the data's max width of 10, so it clamps to the
+    # full range (0, 10).
+    item.zoomAtFraction(0.5, 3.0)
+
+    assert item.viewStartIndex == 0
+    assert item.viewEndIndex == 10
+
+
+def test_pan_shifts_viewport_by_fraction_of_width():
+    item = _eleven_point_item()
+    item.viewStartIndex = 4
+    item.viewEndIndex = 8
+
+    # width=4, pan by 50% of width (2 indices): (4,8) -> (6,10)
+    item.panByFraction(0.5)
+
+    assert item.viewStartIndex == 6
+    assert item.viewEndIndex == 10
+
+
+def test_pan_clamps_at_right_edge_preserving_width():
+    item = _eleven_point_item()
+    item.viewStartIndex = 4
+    item.viewEndIndex = 8
+
+    # width=4, pan by 100% of width (4 indices) would be (8,12); index 12 is
+    # out of bounds (max index 10), so the whole window shifts back by 2 to
+    # stay in bounds while keeping width=4: (6,10).
+    item.panByFraction(1.0)
+
+    assert item.viewStartIndex == 6
+    assert item.viewEndIndex == 10
+
+
+def test_reset_zoom_restores_full_range_sentinel():
+    item = _eleven_point_item()
+    item.viewStartIndex = 3
+    item.viewEndIndex = 7
+
+    item.resetZoom()
+
+    assert item.viewStartIndex == 0
+    assert item.viewEndIndex == -1
+
+
+def test_zoom_noop_when_fewer_than_two_points():
+    from pygui.backend.visualization.graph_quick_item import GraphQuickItem
+
+    item = GraphQuickItem()
+    item.seriesData = [[42.0]]
+
+    item.zoomAtFraction(0.5, 0.5)
+
+    assert item.viewStartIndex == 0
+    assert item.viewEndIndex == -1
+
+
+def test_loading_new_series_while_zoomed_resets_viewport_to_full_range():
+    """Regression: zooming near the tail of a long run, then loading a new
+    (shorter) file, left the stale viewport pointing past the new data --
+    every per-series slice became empty and the chart went blank with a
+    stale Y-range and zeroed-out min/max/avg readouts. New data must reset
+    the viewport to the full range (PopupChartForm.cs's SetData() parity).
+    """
+    from pygui.backend.visualization.graph_quick_item import GraphQuickItem
+
+    item = GraphQuickItem()
+    item.seriesData = [[float(i) for i in range(20)]]
+    item.viewStartIndex = 15
+    item.viewEndIndex = 19
+
+    item.seriesData = [[100.0, 101.0, 102.0, 103.0, 104.0]]
+
+    assert item.viewStartIndex == 0
+    assert item.viewEndIndex == -1
+    assert item.viewMin == pytest.approx(100.0)
+    assert item.viewMax == pytest.approx(104.0)
+    assert item.viewAvg == pytest.approx(102.0)