Add a final sample plot helper

[codecov-commenter]

Codecov Report

Base: 73.21% // Head: 71.86% // Decreases project coverage by -1.35% :warning:

Coverage data is based on head (daf38f2) compared to base (30a74bb). Patch coverage: 28.57% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #22 +/- ## ========================================== - Coverage 73.21% 71.86% -1.36% ========================================== Files 7 7 Lines 224 231 +7 ========================================== + Hits 164 166 +2 - Misses 60 65 +5 ``` | [Impacted Files](https://codecov.io/gh/dstansby/demcmc/pull/22?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Stansby) | Coverage Δ | | |---|---|---| | [src/demcmc/dem.py](https://codecov.io/gh/dstansby/demcmc/pull/22?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Stansby#diff-c3JjL2RlbWNtYy9kZW0ucHk=) | `72.05% <28.57%> (-5.00%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Stansby). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Stansby)

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