I'm a bit confused about the 'free' status of the python library, Plotly.
It says on its website that it's free and open source, but at the same time it says that you'll need a Plotly account if you want to publish the graphs on the web.
I also started to use plotly and I totally agree with you that it is not very clear at first if plotly is free or not. Here are the facts I found about plotly:
In the open-source announcement there is also this information about the other librarys of plotly:
The R, Python, and MATLAB clients for Plotly have always been open-source, while the core graphics layer—plotly.js—was closed-source. Now the entire stack is open-source. Source
The Accounts you can create on their website are just to host graphs online. Like with a free account you can host one private graph, with a professional account you can host unlimited private/public graphs. Link