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

src/ui.cpp

@@ -2908,11 +2908,11 @@ void on_remove_clicked(GtkButton* button, gpointer user) {
       ++removed;
     }
     what = "removed " + std::to_string(removed) + (removed == 1 ? " trace" : " traces") +
-           " from plot " + std::to_string(target->row + 1);
+           " from plot " + app.rows->plot_label(target->row);
   } else {
     const std::string name = r.name[static_cast<std::size_t>(target->slot)];
     remove_trace(app, target->row, target->slot);
-    what = "removed '" + name + "' from plot " + std::to_string(target->row + 1);
+    what = "removed '" + name + "' from plot " + app.rows->plot_label(target->row);
   }
 
   app.relayout_pending = true;
@@ -2990,7 +2990,7 @@ void row_menu_fill(RowMenuState& st) {
 
   if (st.view == RowMenuView::Main) {
     char markup[64];
-    std::snprintf(markup, sizeof(markup), "<b>Plot %d</b>", row + 1);
+    std::snprintf(markup, sizeof(markup), "<b>Plot %s</b>", app.rows->plot_label(row).c_str());
     add_heading(markup);
     add_row("Y axis: auto scale", nullptr, G_CALLBACK(on_y_auto_clicked), &app);
     add_row("Y axis: fixed range...", nullptr, G_CALLBACK(on_y_fixed_clicked), &app);