Add onEnter and onLeave events to the #legend scoped slots for all components concerned:

• triggers the same behavior as hovering / focusing datapoints
• edge cases for series containing multiple datapoints:
  • VueUiScatter, VueUiQuadrant, VueUiWorld: legends list clusters. onEnter should highlight all related datapoints. An additional optional param, provided by the user, would allow the event to act exactly like a datapoint hover, or without showing the datapoint values on x & y.
  • VueUiXy , VueUiStackbar, VueUiStackbar, etc.: onEnter would just trigger some opacity change (in this case, default configuration should feature new attributes to control opacity level).