gui_client: fit x-axis to actual data when less than window size

Track the earliest timestamp across plotted traces (starting from None,
not cutoff) so setXRange uses the first real data point as the left
bound — no empty gap when data doesn't fill the full time window.

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

gui_client.py

@@ -615,6 +615,7 @@ class PlotTab(QWidget):
             if pi.legend:
                 pi.legend.clear()
 
+            x_min = None
             for j, key in enumerate(self._traces[i]):
                 if key not in self._series:
                     continue
@@ -622,6 +623,7 @@ class PlotTab(QWidget):
                 if not series:
                     continue
                 times, values = zip(*series)
+                x_min = times[0] if x_min is None else min(x_min, times[0])
                 label = f"{_trace_label(key)} [{self._units.get(key, '')}]"
                 pw.plot(
                     list(times), list(values),
@@ -629,7 +631,7 @@ class PlotTab(QWidget):
                     name=label,
                 )
 
-            pw.setXRange(cutoff, now, padding=0.02)
+            pw.setXRange(x_min if x_min is not None else cutoff, now, padding=0.02)
 
     # ── cursor tracing ────────────────────────────────────────────────────────