feat: --relative-time selects the counter-derived clock

src/config.hpp

@@ -35,6 +35,11 @@ struct Config {
   /// Number of stacked plots (rows) to start with, 1..`RowSet::kMaxRows`. Only the
   /// starting point: rows are added and removed from the layout editor.
   int plots = 1;
+
+  /// Take the plot's time from the packet counter and the declared sample rate rather than
+  /// from the stream's own time signal, so arrival and producer-clock jitter cannot move a
+  /// sample's x. Absolute stream time is still recorded, for export only (`todo.md` item 2).
+  bool relative_time = false;
 };
 
 /// Parses argv. On `--help` prints usage to stdout and exits(0); on a bad