Determines the group index of this series among sibling Mode2 series sharing the same category (Y) axis. The base implementation in CategorySeries compares via GetXAxis(), which is the *value* axis for vertical series. We must compare via the Y (category) axis instead.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
