proposed addition of a df.agg stacked go.bar example

[rl-utility-man]

### Documentation PR

• [ X] I've seen the doc/README.md file
• [ X ] This change runs in the current version of Plotly on PyPI and targets the doc-prod branch OR it targets the master branch
• If this PR modifies the first example in a page or adds a new one, it is a px example if at all possible
• [ X] Every new/modified example has a descriptive title and motivating sentence or paragraph
• [ X] Every new/modified example is independently runnable
• [ X?] Every new/modified example is optimized for short line count and focuses on the Plotly/visualization-related aspects of the example rather than the computation required to produce the data being visualized
• [ X] Meaningful/relatable datasets are used for all new examples instead of randomly-generated data where possible
• [ N/A] The random seed is set if using randomly-generated data in new/modified examples
• [ N/A ] New/modified remote datasets are loaded from https://plotly.github.io/datasets and added to https://github.com/plotly/datasets
• [ X ] Large computations are avoided in the new/modified examples in favour of loading remote datasets that represent the output of such computations

we want to show the DF agg; I can move some of the other spadework to separate hidden files if needbe

• [ x ] Imports are plotly.graph_objects as go / plotly.express as px / plotly.io as pio
• Data frames are always called df
• fig = <something> call is high up in each new/modified example (either px.<something> or make_subplots or go.Figure)
• [ X ] Liberal use is made of fig.add_* and fig.update_* rather than go.Figure(data=..., layout=...) in every new/modified example
• [ Z] Specific adders and updaters like fig.add_shape and fig.update_xaxes are used instead of big fig.update_layout calls in every new/modified example
• [X ] fig.show() is at the end of each new/modified example
• [ X ] plotly.plot() and plotly.iplot() are not used in any new/modified example
• [ X ] Hex codes for colors are not used in any new/modified example in favour of these nice ones

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

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[Coding-with-Adam]

Thank you for sharing this, @rl-utility-man

[LiamConnors]

I've update my suggestions now @rl-utility-man . This looks good to me. I'm just working on resolving an issue with failing tests on the https://github.com/plotly/graphing-library-docs/ (which is where the docs are built from) and I'll merge this after.

[rl-utility-man]

I've update my suggestions now @rl-utility-man . This looks good to me. I'm just working on resolving an issue with failing tests on the https://github.com/plotly/graphing-library-docs/ (which is where the docs are built from) and I'll merge this after.

Looks great! Much appreciated!

[rl-utility-man]

@LiamConnors Your revisions look great. Is there anything I can do to help get this or #4557 merged? The failing dev_build test reports "ImportError: cannot import name 'Config' from 'pytest'"

[LiamConnors]

@LiamConnors Your revisions look great. Is there anything I can do to help get this or #4557 merged? The failing dev_build test reports "ImportError: cannot import name 'Config' from 'pytest'"

Thanks @rl-utility-man. Sorry for the delay.
I'll merge the latest from doc-prod into your branch. It should have the test fix