The signature of this function should be the following:

string LabelFunction(RangeType type, ValueInfo bottomValue, ValueInfo topValue);

If this function is null, the label is prepared using the default label function:

            string DefaultLabelFunction(RangeType type, ValueInfo bottomValue, ValueInfo topValue)
            {
              switch (type)
              {
                case RangeType.Bounded: return bottomValue.ToString() + " to " + topValue.ToString();
                case RangeType.UnboundedBottom: return "Below " + topValue.ToString();
                case RangeType.UnboundedTop: return bottomValue.ToString() +  " and above";
                default: return "unbounded";
              }
            }
            

Target Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, Windows Vista, Windows Server 2008 family

Reference