Add hover interactivity to Plot.Line and Plot.Area.

Users can use `Interaction.Hover` with `Plot.Line` and `Plot.Area`. The `onHoverOver` callback will be passed the closest point to the cursor by X (Y breaks ties) within 15 pixels.

Close #848.
It does, and in that case the tooltip wouldn't work. Sorting the values defeats the purpose of this algorithm though (it becomes *O(n log n)* instead of *O(log n)*, or even *O(n)* which is what a brute-force algorithm would be).