Custom Sort in DataGrid

I'm doing a custom sort in a datagrid. I'm overriding mousedown and
doing the sort on a hidden column in addition to the column that the
user clicked on.

After the custom sort, I still need to show the little arrow on the
column header that shows that the column is sorted and whether the
sort is ascending or descending.

How to I programatically control the little arrow on the column
header?

Thanks,
Gene H.