Update sankey-diagram.md

[SylwiaOliwia2]

Add colorful links to the Sankey diagram.
#1965

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• I've seen the doc/README.md file
• 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
• Every new/modified example has a descriptive title and motivating sentence or paragraph
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• 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
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• 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)
• Liberal use is made of fig.add_* and fig.update_* rather than go.Figure(data=..., layout=...) in every new/modified example
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• Hex codes for colors are not used in any new/modified example in favour of these nice ones

[emmanuelle]

This is a very nice example, thank you so much @SylwiaOliwia2 ! Could you please add a few explanations:

• change the title to "More complex Sankey diagram with colored links" (titles appear in the sidebar and are indexed by search engines)
• add a comment within the example to explain why you replace magenta (for the opacity)

I also left an inline suggestion.

[emmanuelle]

Thank you very much @SylwiaOliwia2 ! Can you also change the title? If you can't do it now, I think I'll merge this afternoon and we can change the title later, since a question on the user forum would benefit from your example :-) 🎉 (https://community.plotly.com/t/how-to-set-the-color-of-sankey-diagram-lines/36766 - what a timing!)

[SylwiaOliwia2]

On my way to do it @emmanuelle. Just try to update the branch to pass all the tests. Even if it's not necessary, I want to improve my skills. I guess in the evening it will be ready to merge along with the changed title :)

[SylwiaOliwia2]

I've changed the title. The test ci/circleci: python-2.7-plot_ly still fails. If it's not a problem, you can merge.

[emmanuelle]

thanks @SylwiaOliwia2 I'm merging. You would need to merge master (after pulling) to get the commit which fixes the python-2.7 build but it's ok, it is already in master so no problem for merging here. Thanks again!