The proposed future outside lines of streets, parks, playgrounds and other public grounds shown on any plat certified and adopted as hereinbefore provided, may for convenience be shown, wholly or in part, by appropriate symbols on any official map or other map of the city or village: Provided, That showing such lines on any map shall not in and of itself constitute or be deemed to constitute the opening or establishment of any street or the taking or acceptance of any land for any of the aforesaid purposes.