API: Website CORS not allowed

Hi!

I just discovered your package, and I've noticed a bug on the website during the API example, with a "CORS failed" request being returned (https://nivo-api.herokuapp.com/nivo/charts/bar). It seems this isn't the only graph affected.

Also, I found another small visual "bug": when the menu is open, the background tabs are no longer accessible.

Have a good day, team :)