Plot tab: floating tooltip showing trace values at crosshair position

Add PlotTooltip — a frameless child widget that appears next to the
mouse cursor whenever it moves over a plot. It lists each active trace
with a colored bullet, short name, and the nearest y-value (unit
appended). The tooltip is repositioned on every mouse-move event and
clamped to stay within the plot widget bounds. Hidden on mouse-exit and
on clear_data (reconnect).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui_client.py

@@ -390,6 +390,43 @@ class TimeAxisItem(pg.AxisItem):
         return [datetime.fromtimestamp(v).strftime("%H:%M:%S") for v in values]
 
 
+class PlotTooltip(QWidget):
+    """Floating value readout that follows the crosshair cursor."""
+    def __init__(self, parent: QWidget):
+        super().__init__(parent)
+        self.setAttribute(Qt.WA_TransparentForMouseEvents)
+        self.setStyleSheet(
+            "QWidget { background: rgba(18,18,30,215); border: 1px solid #444466;"
+            " border-radius: 3px; }"
+            "QLabel  { color: #ddddee; font-size: 10px; border: none; }"
+        )
+        lay = QVBoxLayout(self)
+        lay.setContentsMargins(6, 4, 6, 4)
+        lay.setSpacing(1)
+        self._labels: list[QLabel] = []
+        self.hide()
+
+    def set_values(self, entries: list):
+        if not entries:
+            self.hide()
+            return
+        while len(self._labels) < len(entries):
+            lbl = QLabel()
+            lbl.setTextFormat(Qt.RichText)
+            self.layout().addWidget(lbl)
+            self._labels.append(lbl)
+        for j, lbl in enumerate(self._labels):
+            if j < len(entries):
+                color, name, val = entries[j]
+                lbl.setText(f'<span style="color:{color};">&#9679;</span> {name}: {val}')
+                lbl.show()
+            else:
+                lbl.hide()
+        self.adjustSize()
+        self.show()
+        self.raise_()
+
+
 class PlotTab(QWidget):
     NUM_PLOTS  = 8
     MAX_TRACES = 4
@@ -428,6 +465,7 @@ class PlotTab(QWidget):
         self._add_combos:   list[QComboBox]       = []
         self._chip_bars:    list[QHBoxLayout]     = []
         self._date_labels:  list[QLabel]          = []
+        self._tooltips:     list[PlotTooltip]     = []
 
         for i in range(self.NUM_PLOTS):
             cell = QWidget()
@@ -506,6 +544,7 @@ class PlotTab(QWidget):
 
             cell_vbox.addWidget(pw)
             self._plot_widgets.append(pw)
+            self._tooltips.append(PlotTooltip(pw))
             plot_grid.addWidget(cell, i // 2, i % 2)
 
         scroll = QScrollArea()
@@ -563,6 +602,8 @@ class PlotTab(QWidget):
         self._redraw_timer.stop()
         self._series.clear()
         self._units.clear()
+        for tt in self._tooltips:
+            tt.hide()
         self._redraw()
 
     def update(self, snapshot: dict):
@@ -689,19 +730,45 @@ class PlotTab(QWidget):
 
     # ── cursor tracing ────────────────────────────────────────────────────────
 
+    def _y_at(self, key: str, x: float):
+        series = self._series.get(key)
+        if not series:
+            return None
+        return min(series, key=lambda p: abs(p[0] - x))[1]
+
     def _on_mouse_move(self, evt, plot_idx: int):
         pos = evt[0]
         pw  = self._plot_widgets[plot_idx]
         vl  = self._vlines[plot_idx]
+        tt  = self._tooltips[plot_idx]
 
         if not pw.sceneBoundingRect().contains(pos):
             vl.setVisible(False)
+            tt.hide()
             return
 
         x = pw.getPlotItem().vb.mapSceneToView(pos).x()
         vl.setPos(x)
         vl.setVisible(True)
 
+        entries = []
+        for j, key in enumerate(self._traces[plot_idx]):
+            y = self._y_at(key, x)
+            if y is None:
+                continue
+            unit = self._units.get(key, "")
+            val_str = (f"{y:g} {unit}").strip()
+            entries.append((self.COLORS[j], _trace_label(key), val_str))
+
+        tt.set_values(entries)
+        if entries:
+            wpos = pw.mapFromScene(pos)
+            tx = int(wpos.x()) + 14
+            ty = int(wpos.y()) - tt.height() // 2
+            tx = min(tx, pw.width()  - tt.width()  - 4)
+            ty = max(4,  min(ty, pw.height() - tt.height() - 4))
+            tt.move(tx, ty)
+
 
 # ── Main window ───────────────────────────────────────────────────────────────