Google公式ガイド
https://developers.google.com/maps/documentation/javascript/heatmaplayer?hl=ja
APIリファレンス
https://developers.google.com/maps/documentation/javascript/reference#HeatmapLayer
HTML + JSのサンプル
https://developers.google.com/maps/documentation/javascript/examples/layer-heatmap
http://jsfiddle.net/EBQQH/
http://www.joyofdata.de/blog/interactive-heatmaps-with-google-maps-api/
Earthquake Mappingを事例にした説明
https://developers.google.com/maps/tutorials/visualizing/earthquakes?hl=ja
Google Maps APIを拡張した独自のAPI？（試してない）
http://www.heatmapapi.com/sample_googlev3.aspx
