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

src/config.cpp

@@ -54,6 +54,9 @@ void print_usage(std::FILE* out, const char* argv0) {
                "  --recv-buffer BYTES  SO_RCVBUF for the receiver socket (default 8388608)\n"
                "  --plots N            stacked plots to start with, 1..8 (default 1); rows are\n"
                "                       added and removed from the layout editor\n"
+               "  --relative-time      take the plot's time from the packet counter and the\n"
+               "                       declared rate, not the stream's time signal; absolute\n"
+               "                       time is then kept for export only\n"
                "  -h, --help           this text\n",
                argv0);
 }
@@ -98,6 +101,8 @@ Config parse_args(int argc, char** argv) {
         fail("--plots must be 1.." + std::to_string(RowSet::kMaxRows) + ": " + v);
       }
       c.plots = static_cast<int>(n);
+    } else if (a == "--relative-time") {
+      c.relative_time = true;
     } else {
       fail("unknown argument: " + a);
     }

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