For speed reasons Surface series draws all its cells without sorting them in Z order. This fast method displays correctly unless the chart is in 3D mode with a large amount of perspective ( ie: tChart1.Aspect.Perspective = 100 )

The Surface HideCells property, when True, sorts all cells by Z order before drawing them. This improves (eliminates) display artifacts due to perspective. Note: Sorting time is proportionally to number of grid cells ( NumXValues by NumZValues ).