I also noticed that there is another property called 'Postion' that is also used to customise the position of the datalabels, that too didnt work. Do let me know if i am doing something incorrect.
Note: Just for your knowledge. In the new release v4.8.0, we have merged Aspose.Grid suite to Aspose.Cells for .NET msi installer as Aspose.Grid would be no longer offered as a separate product now. You need to install the installer (uninstall older one first if you have) and use only Aspose.Cells.dll library in your project for your need. You may also take advantage using Aspose.Cells Grid suite though.