Highlighting closest line

Is it possible to highlight the closest line to the user's mouse position?

For instance like - http://dygraphs.com/gallery/#g/highlighted-series

I've been poking around the source code and documentation and I can see that it's easy to get the X position via `onTrackerChanged` and using `series.AtTime(tracker)`, but from what I can tell there's no way to calculate the closest line based on its Y position? 🤔

Start by reading the source and documentation around onTrackerChanged and series.AtTime(tracker), which the issue identifies as the existing X-position path. Determine how the chart can compare the user's mouse Y position with the available series, then define the completed behavior as highlighting the closest line at that position.

Written by the indexing model from the issue text.