[MRG] fix plot label order for stdout and --csv output

[ctb]

This PR corrects the label order that sourmash plot outputs to stdout and --csv. The actual figure labels were and remain correct.

Plus, and just as importantly, this PR provides a test that actually checks that the order is correct :).

Fixes #1605.

[ctb]

@taylorreiter ready for review. I'll verify that the tests are only breaking because of non-PR related stuff before merging.

[codecov]

Codecov Report

Merging #1821 (b9dbe96) into latest (5bd2e35) will increase coverage by 6.67%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1821      +/-   ##
==========================================
+ Coverage   83.48%   90.15%   +6.67%     
==========================================
  Files         113       87      -26     
  Lines       12183     8482    -3701     
  Branches     1627     1627              
==========================================
- Hits        10171     7647    -2524     
+ Misses       1754      577    -1177     
  Partials      258      258              

Flag	Coverage Δ
python	90.15% <100.00%> (ø)
rust	?

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Impacted Files	Coverage Δ
src/sourmash/fig.py	89.13% <100.00%> (ø)
src/core/src/index/mod.rs
src/core/src/lib.rs
src/core/src/sketch/nodegraph.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/ffi/signature.rs
src/core/src/signature.rs
src/core/src/from.rs
src/core/src/errors.rs
src/core/src/index/linear.rs
... and 17 more

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[taylorreiter]

awesome, LGTM!

[ctb]

🎉