Feat/scatter by size
- [x] closes part of #16827 : makes bubble plots easy with df.plot.scatter(x='col1', y='col2', s='col3') with nice automatic bubble sizing and bubble size legend
- [x] 2 tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
I just rebased what I did a few months ago on top of master. Let me know!
Vincent Hello @VincentAntoine! Thanks for updating the PR.

 - In the file [`pandas/plotting/_core.py`](https://github.com/pandas-dev/pandas/blob/71127594b05bd854eb43166820b71ecf3cf2f12b/pandas/plotting/_core.py), following are the PEP8 issues :

> [Line 3337:44](https://github.com/pandas-dev/pandas/blob/71127594b05bd854eb43166820b71ecf3cf2f12b/pandas/plotting/_core.py#L3337): [E241](https://duckduckgo.com/?q=pep8%20E241) multiple spaces after ','
> [Line 3422:80](https://github.com/pandas-dev/pandas/blob/71127594b05bd854eb43166820b71ecf3cf2f12b/pandas/plotting/_core.py#L3422): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (88 > 79 characters)



 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20572?src=pr&el=h1) Report
> Merging [#20572](https://codecov.io/gh/pandas-dev/pandas/pull/20572?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a77ac2bb9fc47b970378985446e6b983ac51a6b7?src=pr&el=desc) will **decrease** coverage by `0.09%`.
> The diff coverage is `13.95%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20572/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20572?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #20572     +/-   ##
=========================================
- Coverage   91.84%   91.74%   -0.1%     
=========================================
  Files         152      152             
  Lines       49264    49306     +42     
=========================================
- Hits        45246    45237      -9     
- Misses       4018     4069     +51
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.13% <13.95%> (-0.1%)` | :arrow_down: |
| #single | `41.87% <9.3%> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20572?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/20572/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `80.43% <13.95%> (-2.08%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20572/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20572/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.52% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20572?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/20572?src=pr&el=footer). Last update [a77ac2b...7112759](https://codecov.io/gh/pandas-dev/pandas/pull/20572?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 CI is failing.

Can you check the failures (some linting issues) and update? Supplemented by #22403