The IMarkerSymbol::YOffset property does not work correctly with any of the chart symbols: PieChartSymbol, BarChartSymbol, StackedChartSymbol. This property always equals the XOffset value and cannot be set.
There is defective logic in the PieChartSymbol, BarChartSymbol, and StackedChartSymbol implementations of IMarkerSymbol.
There is no workaround for this problem. This problem has been resolved in version 9.1.