signals: label plots by row and cell, drop onto a cell

src/ui_signals.cpp

@@ -554,7 +554,7 @@ bool assign_signal_to_row(App& app, std::size_t signal, int row) {
   if (slot < 0) {
     // Say what to do about it. A refusal a reader cannot act on is only half an answer: the two ways
     // out are another plot or another row.
-    notice("plot " + std::to_string(row + 1) +
+    notice("plot " + app.rows->plot_label(row) +
            " holds four signals already: drop it on another plot, or add a row with the layout "
            "editor (L)");
     return false;
@@ -567,7 +567,7 @@ bool assign_signal_to_row(App& app, std::size_t signal, int row) {
   schedule_layout_save(app);
   app.relayout_pending = true;
   rebuild_signal_panels(app);  // the row's colour chip now has a trace to name
-  notice("plotting '" + name + "' on plot " + std::to_string(row + 1));
+  notice("plotting '" + name + "' on plot " + app.rows->plot_label(row));
   return true;
 }