[dataviz] feat: add graph Active contributors grouped by age

[NatNgs]

related issues #1726

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Add user-growth plot in Tournesol's streamlit app

Added a new plot in Streamlit, displaying the number of contributors, new users and active users in one line plot.

"Active" users are defined as having done at least one comparison on the week or before, and having done at least one comparison on the week or after.

Checklist

• [x] I added the related issue(s) id in the related issues section (if any)
• [x] I described my changes and my decisions in the PR description
• [x] I read the development guidelines of the CONTRIBUTING.md
• [x] The tests pass and have been updated if relevant
• [ ] The code quality check pass

❤️ Thank you for your contribution!

[NatNgs]

Updated graph after comments on discord; updated graph now look like this:

• Number of categories (=colors, see legend) can be modified easily as needed
• Choice of colors (as the rainbow gradient here) to be confirmed
• Also not sure about how I labelled it ("Age of community", "First comparison date = last comparison date")

[GresilleSiffle]

Hi @NatNgs

The new graph is really cool :ok_hand:

I ran black -l 99 on the file to make it more readable, and updated some comments and texts to make them more explicit:

• the title is now "Active contributors grouped by age", to introduce the notion of active contributors;
• there is an info box explaining what is an active contributor to avoid any confusion;
• the label of the Y axis is now "Active contributors" to match the title.

The colours and the groups look good to me for now. Let's check with the rest of the team.

Also not sure about how I labelled it ("Age of community", "First comparison date = last comparison date")

I haven't updated "First comparison date = last comparison date" yet, but I'll think about it.