[C#]	public double getZAtValue(double x, double y);
[VB]	Public Function getZAtValue(x As Double, y As Double) As Double

Gets the z value at the specify (x, y) data values.

Note: This method should be used only after ChartDirector has output the chart image, or after BaseChart.layout has been called.

The z value at the sepcified (x, y) data coordinate, or NoValue if the point lie outside the contour region.