Fix cargo payment graph displaying vanilla vaules despite doubled tim…

…e range (fixes #38)

src/graph_gui.cpp

@@ -1299,7 +1299,7 @@ struct PaymentRatesGraphWindow : ExcludingCargoBaseGraphWindow {
 		for (const CargoSpec *cs : _sorted_standard_cargo_specs) {
 			this->colours[i] = cs->legend_colour;
 			for (uint j = 0; j != this->num_on_x_axis; j++) {
-				this->cost[i][j] = GetTransportedGoodsIncome(10, 20, j * 4 + 4, cs->Index());
+				this->cost[i][j] = GetTransportedGoodsIncome(10, 20, 2 * (j * 4 + 4), cs->Index());
 			}
 			i++;
 		}