diff --git a/src/graph_gui.cpp b/src/graph_gui.cpp
index 543f7a2055..e54f736238 100644
--- a/src/graph_gui.cpp
+++ b/src/graph_gui.cpp
@@ -1923,12 +1923,13 @@ struct TownCargoGraphWindow : BaseCargoGraphWindow {
 		const Town *t = Town::Get(this->window_number);
 
 		this->data.clear();
+		this->data.reserve(
+			2 * std::ranges::count_if(t->supplied, &IsValidCargoType, &Town::SuppliedCargo::cargo));
+
 		for (const auto &s : t->supplied) {
 			if (!IsValidCargoType(s.cargo)) continue;
 			const CargoSpec *cs = CargoSpec::Get(s.cargo);
 
-			this->data.reserve(this->data.size() + 2);
-
 			DataSet &produced = this->data.emplace_back();
 			produced.colour = cs->legend_colour;
 			produced.exclude_bit = cs->Index();