New y-axis mode for tooltips

[GitAndKnife]

In my case i need to show tooltip data on same y position. I made a solution for my case, with changing code for core.controller::getElementsAtEvent(e), but i suppose y-axis mode will be useful for others to. Just a suggestion.

[etimberg]

That's an interesting use case. I'd be curious to hear if others are looking for it as well

[panzarino]

Could you provide a demo on a website such as JS Bin, JS Fiddle, or Codepen? This functionality looks interesting and I would like to check it out.

[GitAndKnife]

y-axis
Added to code pen. Right points are selected. But just one is show in tooltip. I modified as mentioned getElementsAtEvent method.

[etimberg]

Implemented in #3400