Excel Pivot Tables
https://support.office.com/en-us/article/Calculate-values-in-a-PivotTable-report-11f41417-da80-435c-a5c6-b0185e59da77#ID0EAABAAA=2007
https://exceljet.net/things-to-know-about-excel-pivot-tables
https://exceljet.net/blog/5-pivot-tables-you-probably-havent-seen-before
http://stackoverflow.com/questions/11720031/count-number-of-times-a-date-occurs-and-make-a-graph-out-of-it
https://exceljet.net/things-to-know-about-excel-pivot-tables
https://exceljet.net/blog/5-pivot-tables-you-probably-havent-seen-before
http://stackoverflow.com/questions/11720031/count-number-of-times-a-date-occurs-and-make-a-graph-out-of-it