[python-package] Fix mypy checks in plotting.py

[borchero]

Motivation

mypy currently fails with 36 errors in 6 files. One of these files is plotting.py.

Changes

This PR fixes the typing issues in plotting.py, eliminating 1 error and 1 file with errors.

[jameslamb]

Thanks for working on this! Similar to my comments on #6898 and #6900, can you please add the warnings from mypy to the PR description, so we know what specific problem this is attempting to fix?

[jameslamb]

can you please add the warnings from mypy to the PR description, so we know what specific problem this is attempting to fix?

I want to move this forward, so I ran mypy locally and looked at CI logs myself.

Neither showed anything about this line. In a recent PR CI run of the lint task, there was only 1 mypy finding about plotting.py:

python-package/lightgbm/plotting.py:143: error: Argument 2 to "text" of "Axes" has incompatible type "signedinteger[Any]"; expected "float"  [arg-type]

(build link)

That doesn't look related to the changes in this PR.

I'm closing this. In the future, please:

• include the text of the warnings / errors you are attempting to address
• link to any related issues (in this case, [ci] warnings from mypy #3867)