Add tests for `plot` module #61

Job	Run time
Determine if CI should be skipped	3s
Unit tests (ubuntu-latest, 3.8)	12m 32s
Unit tests with minimum dependency versions (ubuntu-latest, 3.8)	12m 48s
Unit tests (ubuntu-latest, 3.9)	13m 1s
Unit tests (ubuntu-latest, 3.10)	11m 37s
Unit tests (macos-latest, 3.8)	15m 15s
Unit tests (macos-latest, 3.9)	14m 5s
Unit tests (macos-latest, 3.10)	12m 51s
Upload coverage	0s
	1h 32m 12s

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