I suggest to add the option to compare each point.y with another poin.y at the same point.x (for example, percent with respect at the first serie in each point), not just compare with the first value in the serie, for stock charting.
Thanks for sharing the idea. Could you provide more details? At this moment you can compare visually values e.g. using `split` or `shared` tooltip: https://jsfiddle.net/BlackLabel/e8poqvgm/
In tooltip, you can compare directly the same points, for example: https://jsfiddle.net/BlackLabel/e8poqvgm/1/
If you have any details (equations, examples etc.), please provide them. The better description, the better solution we can provide. Thanks!