• Use finite differences to estimate derivative rather than splines
  (which are conceptually much more complicated).
• Include a "constant error" case, which shows that the band does have
  constant width in that case.
• Force the axes aspect to 1 (so that constant width actually appears
  constant).
• Use np.full() to set up the codes array.

old version, for reference:https://matplotlib.org/devdocs/gallery/lines_bars_and_markers/curve_error_band.html
new (second) plot:

PR Summary

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings and runflake8 --docstring-convention=all).
• New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).