Break lines when yValues are missing for consecutive indexes

[liuxuan30]

There should be demand when we add xIndex but no entry, the line should be truncated, not goes to next entry.

[danielgindi]

Currently the behaviour in line charts for missing y values is "undefined". We are going to define it soon, and possibly allow more control over that decision.

[liuxuan30]

I tried to implement mine last week, it could break the lines. But for the filling function, it's still buggy. The filling rect is quite complicated when involving negative values. It seems the filling rect will always use y=0 axis line, so when there is only negative values, it fill the rect above line, below y=0 axis.

[noais]

@liuxuan30 have you some news about this issue?

[liuxuan30]

Well I could break the lines, but the rect filling part is still buggy and I gave up, too many corner cases for my implementations and I don't have much time to think carefully.

[noais]

Do you have this alterations on your branch? Maybe I can try to help you.

[liuxuan30]

I don't have yet, I just tried in my sub class, you can have your implementations and better than mine :)

Breaking the lines is simple. Just start another moveToPoint loop and close previous path properly. I feel the hard part is the filling function. Consider if there are all negative values, and the y axis is at the top... and half negative and half postive..

[noais]

This week end I'm going to try implement this feature.

[noais]

Pull request made #335. Can you have a look I give me your opinion please.