Hot regions are the areas on the chart image that define where tool tips are displayed.

Tooltip HotRegions are defined differently for curved chart family and can be further categorized in Smooth curve ( spline ) and non smooth curved  ( line) chart types .

For Line chart types tool tip hot region is divided between two consecutive data points and the last data point in the order of connectivity owns the Hot Region.

In the following image , the hot region between the two data points is equally divided among two data points and the first part of region is owned by first data point and the second part of region is owned by second data point in connection.

In the following image you see two data points but since the Chart type is "spline" the region between the two data points including the second data point is Hot Region for second data point.

To change the hot regrion of your chart to act like the hot region of a line chart, set the TooltipOffet Custom Attribute to True.

[C#]
Chart1.Series[0]["TooltipOffset"] = "True"; 

[VB.NET]
Chart1.Series[0]["TooltipOffset"] = "True"; 

Note: The TooltipOffset Attribute can only be set with a Spline, Area, SplineArea and Range Column chart.