They look quite nice. But I would prefer that the default way of initializing the dataset is by loading a HTML table. This way content would always be accessible, even without Javscript and also by crawlers like Google.
There's nothing stopping you from doing that with a pinch of javascript.
Of course :) I just think that doing it from a HTML table is best practice and it's always nice when a library promotes these. Most developers simply go: 1) I want chart 2) Chart needs JSON 3) I give chart JSON