fix chan swap

rfg_adc_plotter/io/sweep_parser_core.py

@@ -500,18 +500,27 @@ class SweepAssembler:
             self._reset_current()
             if event.ch is not None:
                 self._cur_channel = int(event.ch)
-                self._cur_channels.add(int(event.ch))
             return packet
 
+        point_ch = int(event.ch)
+        packet: Optional[SweepPacket] = None
         if self._cur_channel is None:
-            self._cur_channel = int(event.ch)
-        self._cur_channels.add(int(event.ch))
+            self._cur_channel = point_ch
+        elif point_ch != self._cur_channel:
+            if self._xs:
+                # Never mix channels in a single sweep packet: otherwise
+                # identical step indexes can overwrite each other.
+                packet = self.finalize_current()
+                self._reset_current()
+            self._cur_channel = point_ch
+
+        self._cur_channels.add(point_ch)
         self._xs.append(int(event.x))
         self._ys.append(float(event.y))
         if event.aux is not None:
             self._aux_1.append(float(event.aux[0]))
             self._aux_2.append(float(event.aux[1]))
-        return None
+        return packet
 
     def finalize_current(self) -> Optional[SweepPacket]:
         if not self._xs: