add num_minor_intervals helper

[t-bltg]

For #4542.

[codecov]

Codecov Report

Base: 89.21% // Head: 89.22% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (e806496) compared to base (3940347).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4548   +/-   ##
=======================================
  Coverage   89.21%   89.22%           
=======================================
  Files          40       40           
  Lines        7734     7738    +4     
=======================================
+ Hits         6900     6904    +4     
  Misses        834      834           

Impacted Files	Coverage Δ
src/axes.jl	90.61% <100.00%> (+0.08%)	⬆️

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