[olq_plo]

Labels are metadata. You can attach arbitrary metadata to each axis or no metadata at all. The metadata is sometimes useful to keep track what each axis range was representing. For example, if you load a serialized histogram into your program that someone else generated.

Here is an example which demonstrates the usefulness of labels: https://raw.githubusercontent.com/boostorg/histogram/develop...

The design choice of using operator() or a fill() method was hotly debated. In the bigger picture of things, operator() is better because then 1D histogram have the same fill syntax as the accumulators from Boost::accumulators.