Check this doc http://docs.oracle.com/cd/E21764_01/bi.1111/e16452/apndx_ui_chngs.htm
as per that:
"Measure-based sorts are not implemented completely for pivot tables and graphs in 11g. Graphs do not provide user-interface elements to emulate measure-based sorts. This is a restriction in 11g, and you can use workarounds to resolve sorting issues."
If helps pls mark