fix histogram operator

[StRigaud]

closes #280

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.92%. Comparing base (9fd6f78) to head (db72dd0).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #281   +/-   ##
=======================================
  Coverage   87.92%   87.92%           
=======================================
  Files          20       20           
  Lines        1607     1607           
=======================================
  Hits         1413     1413           
  Misses        194      194           

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