Thanks a lot Ines.
It's a nice example.
For other people looking into it, I could not find a documentation on the static/index.html but I ran into this comment which explains it all : https://support.prodi.gy/t/removing-meta-div-from-display-and-custom-css/1028/6 .
Fair question about just sending an svg or an image data. That's what I am currently doing and using altair. So you bullseyed it. 
In a more complete version, I want my users to be able to browse the chart, interact with some specifics dots and probably select those and add them the answer.