Let the waterfall plot thin its x labels

[TimoDiepers]

What

An inventory spanning decades gives plot_characterized_inventory_as_waterfall one bar — and one rotated label — per time step, which stops being readable well before the axis runs out of room. A 50-year use phase produces 51 of them.

xtick_interval=5 labels every fifth step and hides the rest:

plot_characterized_inventory_as_waterfall(tlca, xtick_interval=5)

The default stays None, i.e. every step labelled, so nothing changes for existing code.

The static and prospective columns are not time steps, so they are skipped by the counter and keep their labels whatever the interval is.

Tests

Four cases in tests/test_utils.py against a stub carrying only what the function reads: no interval labels all twelve years, 5 gives 2025/2030/2035, 4 gives 2025/2029/2033, and a fourth asserts static and prospective survive alongside the thinned years.

Worth knowing for anyone writing similar tests: matplotlib drops invisible labels from get_xticklabels() once the figure is drawn, so the assertions compare the visible label texts rather than counting hidden ones.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.92%. Comparing base (e567e20) to head (56b874f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
+ Coverage   83.75%   84.92%   +1.17%     
==========================================
  Files          16       16              
  Lines        3760     3767       +7     
==========================================
+ Hits         3149     3199      +50     
+ Misses        611      568      -43     

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