I believe that it is a best practive to add these kind of libraries to their own Space / Extension and make them available via the Forge. There are so many great js libraries that could be added to the system by default, but by adding all libraries by default it would slow down the loading of the applications.

Agree with Remco.

Adding all kinds of js to OS will only slow down applications when you don't need the js for your application.

I agree it should not be implemented
(why choose D3 and not highcharts for example)

It is an interesting library though, you should ask in the forums if someone already made it and if it could be placed in the Forge.

In version 8 we've actually bundled a powerful charting library based on Highcharts. 

I agree with the guys that d3 should not be included IN the platform, but instead as a component in the Forge. Actually, at this moment I believe there is already a component in the Forge (Discovery) that uses d3 to show some interesting data visualizations out of your application's architecture. Check them out

D3 is the best library I found to build interactive data visualization, and that is the reason I used it for Discovery. I love it in many aspects, but it is also a challenging one in terms of abstraction. You need to code and think using functional programming, and when it doesn't work, it is painfull to fix it. Instead of packaging the library as such, I would prefer to package some of the patterns built on top of D3 as widgets like sankey diagrams (in the above picture) as components in the Forge.