Fix virtual channels not appearing in Plot Builder

Root cause: PlotWindow deepcopied _chart._cfg on init, so refresh_channels()
operated on a stale copy. Also, when _win_plot was None, _on_derived_changed
did nothing to prepare the config for when it opened.

Fix:
- refresh_channels() now owns the derived-pane sync: adds panes for new
  virtual channels (updating BSP tree), removes panes for deleted ones
- _open_plot always passes current _chart._cfg reference then calls
  refresh_channels() so derived panes sync on every open
- _on_derived_changed syncs cfg reference before calling refresh_channels()

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

1 files changed, 6 insertions, 2 deletions

diff --git a/ui/main_window.py b/ui/main_window.py
index 5078a60..ebc7463 100644
--- a/ui/main_window.py
+++ b/ui/main_window.py
@@ -352,9 +352,10 @@ class MainWindow(QMainWindow):
                                         self._chart._cfg, self)
             self._win_plot.layout_applied.connect(self._chart.apply_layout)
             self._win_plot.closed.connect(lambda: self._btn_plot.setChecked(False))
+            self._win_plot.refresh_channels()   # sync any derived channels
         else:
             self._win_plot.cfg = self._chart._cfg
-            self._win_plot._populate()
+            self._win_plot.refresh_channels()
         self._show_win(self._win_plot, "below")
 
     def _open_settings(self):
@@ -507,7 +508,10 @@ class MainWindow(QMainWindow):
 
     def _on_derived_changed(self):
         self._chart.refresh()
-        if self._win_plot: self._win_plot.refresh_channels()
+        if self._win_plot:
+            # Ensure plot window works on current chart cfg, then sync derived panes
+            self._win_plot.cfg = self._chart._cfg
+            self._win_plot.refresh_channels()
 
     # ── Run / Log ─────────────────────────────────────────────────────────