How to hold two points on a chart and get the difference between them?

[Manas-Nagelia]

Hey there.

First of all, great work on the library! Wonderful library.

I have a question, how do I go about if I want to hold two points between a chart (line or area), and I'll see the percent change between them? Like if you were to type a stock in google finance, and hold two points together, you'll see what I mean. Below are two images, one from Google Finance, and the other from Apple Stocks. I believe the Apple Stocks one (second image) does the effect better.

Thanks.

[Manas-Nagelia]

This question is also in an issue: #308

[Manas-Nagelia]

I know that this feature isn't implemented yet (in the issue they said they have it on their roadmap), but is there any way to manually code this feature? Thanks!