Plot tab: HH:MM:SS x-axis ticks, date label in control bar

Replace pg.DateAxisItem with a custom TimeAxisItem that formats ticks
as HH:MM:SS, eliminating locale-dependent day/date prefixes on the axis.

Add a per-plot date label (right-aligned in the control bar) that shows
the date range of the visible window — single date when the view stays
within one day, "YYYY-MM-DD – YYYY-MM-DD" when it spans midnight.

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

gui_client.py

@@ -384,6 +384,12 @@ class DashboardTab(QWidget):
 
 # ── Plot tab ──────────────────────────────────────────────────────────────────
 
+class TimeAxisItem(pg.AxisItem):
+    """X-axis that shows HH:MM:SS, suppressing any date prefix."""
+    def tickStrings(self, values, scale, spacing):
+        return [datetime.fromtimestamp(v).strftime("%H:%M:%S") for v in values]
+
+
 class PlotTab(QWidget):
     NUM_PLOTS  = 8
     MAX_TRACES = 4
@@ -421,6 +427,7 @@ class PlotTab(QWidget):
         self._proxies:      list = []
         self._add_combos:   list[QComboBox]       = []
         self._chip_bars:    list[QHBoxLayout]     = []
+        self._date_labels:  list[QLabel]          = []
 
         for i in range(self.NUM_PLOTS):
             cell = QWidget()
@@ -465,12 +472,18 @@ class PlotTab(QWidget):
             ctrl_row.addLayout(chip_bar)
             ctrl_row.addStretch()
 
+            date_lbl = QLabel("")
+            date_lbl.setStyleSheet("color: #666688; font-size: 10px;")
+            date_lbl.setAlignment(Qt.AlignRight | Qt.AlignVCenter)
+            self._date_labels.append(date_lbl)
+            ctrl_row.addWidget(date_lbl)
+
             cell_vbox.addWidget(ctrl)
 
             # plot
             pw = pg.PlotWidget(
                 background="#12121e",
-                axisItems={"bottom": pg.DateAxisItem(orientation="bottom")},
+                axisItems={"bottom": TimeAxisItem(orientation="bottom")},
             )
             pw.showGrid(x=True, y=True, alpha=0.3)
             pw.addLegend(offset=(0, 0), labelTextSize="8pt")
@@ -663,7 +676,16 @@ class PlotTab(QWidget):
                     name=label,
                 )
 
-            pw.setXRange(x_min if x_min is not None else cutoff, now, padding=0.02)
+            x_lo = x_min if x_min is not None else cutoff
+            pw.setXRange(x_lo, now, padding=0.02)
+
+            d_lo = datetime.fromtimestamp(x_lo).date()
+            d_hi = datetime.fromtimestamp(now).date()
+            if d_lo == d_hi:
+                date_str = d_lo.strftime("%Y-%m-%d")
+            else:
+                date_str = f"{d_lo.strftime('%Y-%m-%d')} – {d_hi.strftime('%Y-%m-%d')}"
+            self._date_labels[i].setText(date_str)
 
     # ── cursor tracing ────────────────────────────────────────────────────────