jQuery Grid Plugin - jqGrid

I see , while multi-sort, I cannot manipulate sequence of column. 

For Example, I have a table which has 3 cols and their name are :  Col A, Col B, Col C.

I click Col B, and then Col A. The sort is always 'Col A asc, Col B'. But, I hope sort is 'Col B asc, Col A'.

How can I manipulate the sequence of column?

I try to set the value of sortname in onSortCol, but it doesnot work.