Determines whether the series should look for the nearest point
in both dimensions or just the x-dimension when hovering the series.
Defaults to 'xy'
for scatter series and 'x'
for most other
series. If the data has duplicate x-values, it is recommended to
set this to 'xy'
to allow hovering over all points.
Applies only to series types using nearest neighbor search (not direct hover) for tooltip.