DOC: EX03 fixes on dataframe.plot methods: hexbin, line
- [x] xref #56804 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

@alpakpinar I don't think this should be marked to close https://github.com/pandas-dev/pandas/issues/56804 yet, 
There are still a few methods remaining to be fixed. Would you consider updating it to xref format?

I've been using this for my PRs: 

- [ ] xref https://github.com/pandas-dev/pandas/issues/56804 @jordan-d-murphy thanks! I updated the description of this pull request and also #56898  Thanks @alpakpinar 