Various enhancements for plot command

[yarikoptic]

• 3729906 - (HEAD -> enh-plot, origin/enh-plot) fix(plot,test): simply point to non-existing file and fixup testing (3 minutes ago) [Yaroslav Halchenko]
• be4e26c - fix(plot): delay import of matplotlib and numpy (15 minutes ago) [Yaroslav Halchenko]
• 37a4034 - feat(plot): do not "hardcode" .png . --output filename should include extension (15 minutes ago) [Yaroslav Halchenko]
• 457e54b - docs: adjust descriptions/help to uniformly end with . and some wording (15 minutes ago) [Yaroslav Halchenko]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.32%. Comparing base (98d7f34) to head (26f4f0e). Report is 7 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #217 +/- ## ======================================= Coverage 95.32% 95.32% ======================================= Files 4 4 Lines 620 620 Branches 68 68 ======================================= Hits 591 591 Misses 15 15 Partials 14 14 ```

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