Sticky tracking of mouse events. When true, the mouseOut event
on a series isn't triggered until the mouse moves over another
series, or out of the plot area. When false, the mouseOut event on
a series is triggered when the mouse leaves the area around the
series' graph or markers. This also implies the tooltip. When
stickyTracking is false and tooltip.shared is false, the tooltip
will be hidden when moving the mouse between series.