Fix typing for test_plot.py (#9234) #13329

Job	Run time
detect ci trigger	4s
Event File	3s
ubuntu-latest py3.9	8m 25s
ubuntu-latest py3.12	9m 32s
macos-latest py3.9	5m 0s
macos-latest py3.12	6m 48s
windows-latest py3.9	13m 0s
windows-latest py3.12	19m 5s
ubuntu-latest py3.9 bare-minimum	1m 38s
ubuntu-latest py3.9 min-all-deps	6m 36s
ubuntu-latest py3.11 all-but-dask	6m 20s
ubuntu-latest py3.12 flaky	1m 20s
	1h 17m 51s

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