https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.datavisualization.charting.datapointcustomproperties.legendtooltip?view=netframework-4.8.1
DataPointCustomProperties.LegendToolTip Property (System.Web.UI.DataVisualization.Charting) |...
Gets or sets the tooltip of the item in the legend.
property systemweb uidatavisualizationcharting