D3のInterpolation機能を使って、地図上のパスを滑らかに補完する。
Smooth pathのD3 ver.4版
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Updated missing url https://maps.googleapis.com/maps/api/js?key=AIzaSyA8X-e3Naz_Pu50cP8bdDQUZukKrXMqDGs&libraries=visualization to https://maps.googleapis.com/maps/api/js?key=aizasya8x-e3naz_pu50cp8bddquzukkrxmqdgs&libraries=visualization
https://maps.googleapis.com/maps/api/js?key=AIzaSyA8X-e3Naz_Pu50cP8bdDQUZukKrXMqDGs&libraries=visualization
https://unpkg.com/d3@4.12.2/build/d3.min.js