Plot tab: auto-zoom Y axis on first draw after connect / trace change

Track a _needs_autozoom flag per plot. Set True at startup, on
reconnect (clear_data), and when a trace is added or removed. In
_redraw, when the flag is set and visible data exists, fit the Y range
to the data (with 10% margin) then clear the flag — subsequent live
updates leave the user's manual Y zoom undisturbed.

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

gui_client.py

@@ -466,6 +466,7 @@ class PlotTab(QWidget):
         self._chip_bars:     list[QHBoxLayout]     = []
         self._date_labels:   list[QLabel]          = []
         self._tooltips:      list[PlotTooltip]     = []
+        self._needs_autozoom: list[bool]           = [True] * self.NUM_PLOTS
 
         for i in range(self.NUM_PLOTS):
             cell = QWidget()
@@ -563,6 +564,7 @@ class PlotTab(QWidget):
             return
         traces.append(key)
         self._add_chip(plot_idx, key, len(traces) - 1)
+        self._needs_autozoom[plot_idx] = True
         self._save_settings()
         self._redraw()
 
@@ -584,6 +586,7 @@ class PlotTab(QWidget):
             return
         self._traces[plot_idx].remove(key)
         self._rebuild_chips(plot_idx)
+        self._needs_autozoom[plot_idx] = True
         self._save_settings()
         self._redraw()
 
@@ -604,6 +607,7 @@ class PlotTab(QWidget):
         self._units.clear()
         for tt in self._tooltips:
             tt.hide()
+        self._needs_autozoom = [True] * self.NUM_PLOTS
         self._redraw()
 
     def update(self, snapshot: dict):
@@ -702,6 +706,7 @@ class PlotTab(QWidget):
                 pi.legend.clear()
 
             x_min = None
+            y_all: list[float] = []
             for j, key in enumerate(self._traces[i]):
                 if key not in self._series:
                     continue
@@ -710,6 +715,7 @@ class PlotTab(QWidget):
                     continue
                 times, values = zip(*series)
                 x_min = times[0] if x_min is None else min(x_min, times[0])
+                y_all.extend(values)
                 label = f"{_trace_label(key)} [{self._units.get(key, '')}]"
                 pw.plot(
                     list(times), list(values),
@@ -720,6 +726,12 @@ class PlotTab(QWidget):
             x_lo = x_min if x_min is not None else cutoff
             pw.setXRange(x_lo, now, padding=0.02)
 
+            if y_all and self._needs_autozoom[i]:
+                y_min, y_max = min(y_all), max(y_all)
+                margin = max((y_max - y_min) * 0.1, 1.0)
+                pw.setYRange(y_min - margin, y_max + margin, padding=0)
+                self._needs_autozoom[i] = False
+
             d_lo = datetime.fromtimestamp(x_lo).date()
             d_hi = datetime.fromtimestamp(now).date()
             if d_lo == d_hi: