Event which is raised before tooltip is updated.
Return false in order to cancel updating and hide tooltip.
Function takes arguments evt and ui.
Use ui.owner to obtain reference to chart widget.
Use ui.text to obtain html of tooltip. Value of that member can be modified. If modified value is null or empty string, then current content of tooltip keeps old value.
Use ui.item to obtain reference to item. Value of that member can be modified or replaced by custom item.
Use ui.x to obtain left position of tooltip in pixels relative to widget. Value of that member can be modified.
Use ui.y to obtain top position of tooltip in pixels relative to widget. Value of that member can be modified.
Use ui.element to obtain reference to jquery object which represents tooltip. Value of that member can be replaced by custom element.
Target Platforms: Windows 8.1, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
