Every execution ever performed in the USA. You can see a marked spike in the graph in 1942-1944 when 7 Murder Incorporated members all went to to have a visit with "Old Sparky" at Sing-Sing.
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I wonder if Mendy Weiss, Happy Maione, Louis "Lepke" Buchalter, Harry "Pittsburgh Phil" Strauss, Frank "The Dasher" Abbandando, Louis Capone, and Martin "Bugsy" Goldstein ever thought that their trip to the chair would be the spike in the period of the most executions in US History.

Interesting graph to say the least.