feat: the measurements pane can show how each reading varies, and remembers that it was asked to

src/layout_store.cpp

@@ -309,6 +309,8 @@ bool load_layout(const std::string& path, LayoutState& out) {
           plot_count = std::max(0, std::stoi(value));
         } catch (...) {
         }
+      } else if (key == "measure_stats") {
+        st.measure_stats = value == "true";
       } else if (key == "multicast") {
         st.multicast = strip_quote(value);
       } else if (key == "interface") {
@@ -431,6 +433,7 @@ bool save_layout(const std::string& path, const LayoutState& state) {
     }
     o << "]\n";
   }
+  o << "measure_stats = " << (state.measure_stats ? "true" : "false") << "\n";
   for (const auto& row : state.rows) {
     o << "\n[[plot]]\nsignals = [";
     for (std::size_t i = 0; i < row.signals.size(); ++i) {