feat: Add unit tests for the plot module #363

Job	Run time
quantumespresso	26m 30s
documentation	44s
black	13s
arithmetic	2m 20s
unittest_matrix (ubuntu-latest, 3.12)	49s
unittest_matrix (ubuntu-latest, 3.11)	54s
pip_check	52s
unittest_matrix (macos-latest, 3.12)	1m 51s
nfdi4ing	7m 6s
	41m 19s

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