COMPAT: mpl 3.0
Closes #22790 
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/plotting/_compat.py`](https://github.com/TomAugspurger/pandas/blob/b59fd148979ab7c2c1b7a233c9351fbe33ea5318/pandas/plotting/_compat.py) !

 - There are no PEP8 issues in the file [`pandas/plotting/_core.py`](https://github.com/TomAugspurger/pandas/blob/b59fd148979ab7c2c1b7a233c9351fbe33ea5318/pandas/plotting/_core.py) !

 - There are no PEP8 issues in the file [`pandas/tests/plotting/common.py`](https://github.com/TomAugspurger/pandas/blob/b59fd148979ab7c2c1b7a233c9351fbe33ea5318/pandas/tests/plotting/common.py) !

 - There are no PEP8 issues in the file [`pandas/tests/plotting/test_datetimelike.py`](https://github.com/TomAugspurger/pandas/blob/b59fd148979ab7c2c1b7a233c9351fbe33ea5318/pandas/tests/plotting/test_datetimelike.py) !

 The test `pandas/tests/plotting/test_datetimelike.py::TestTSPlot::test_high_freq` takes 123s for me locally.  That seems excessive. I wonder if there was a performance regression there, or whether we can make the test smaller.
 https://github.com/pandas-dev/pandas/commit/5fbffe4d1cd8b422119eb2b86c8a3f58e940f18c was the original commit (6 years ago). Nothing in that looks like it depends on the number of points, just the freq, so I'm going to reduce the number to 100 (finishes in 0.5s locally with that). @jreback good to merge on green, to get master passing again? (aside from https://github.com/pandas-dev/pandas/issues/22871 which is failing our strict warnings build) yep # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22870?src=pr&el=h1) Report
> Merging [#22870](https://codecov.io/gh/pandas-dev/pandas/pull/22870?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d115900f4f80d2f9016b41041bde1564980415b3?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22870/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22870?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22870      +/-   ##
==========================================
- Coverage   92.19%   92.18%   -0.01%     
==========================================
  Files         169      169              
  Lines       50827    50830       +3     
==========================================
  Hits        46860    46860              
- Misses       3967     3970       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.37% <33.33%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22870?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/22870/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb21wYXQucHk=) | `91.3% <100%> (+0.39%)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/22870/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.35% <100%> (-0.2%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22870?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22870?src=pr&el=footer). Last update [d115900...2ca4963](https://codecov.io/gh/pandas-dev/pandas/pull/22870?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Given that `0.23.x` is still active, I would recommend backporting CI fixes like this.  When I attempted to backport an actual regression last time, I ran into numerous CI-related issues unrelated to the original fix.