gui_client: clear plot data on connect/reconnect

Add PlotTab.clear_data() which resets _series and _units, then redraws.
Called in MainWindow._connect() before opening the socket so stale data
from a previous session never mingles with fresh history from the server.

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

gui_client.py

@@ -518,7 +518,12 @@ class PlotTab(QWidget):
         for j, key in enumerate(self._traces[plot_idx]):
             self._add_chip(plot_idx, key, j)
 
-    # ── public update called on every new snapshot ────────────────────────────
+    # ── public API ────────────────────────────────────────────────────────────
+
+    def clear_data(self):
+        self._series.clear()
+        self._units.clear()
+        self._redraw()
 
     def update(self, snapshot: dict):
         ts_raw = snapshot.get("ts", "")
@@ -680,6 +685,7 @@ class MainWindow(QMainWindow):
         self._record_count = 0
 
     def _connect(self, host: str, port: int):
+        self._plot.clear_data()
         err = self._reader.connect_to(host, port)
         if err:
             self._connector.set_disconnected(err)