What would be the easiest or recommended way for making an NSOutlineView indent multiple columns? By default, it only indents the outline column; and as far as I know there is no built-in support for making it indent other columns.

I have an NSOutlineView which shows a comparison between two sets of hierarchical data. For visual appeal, if some item in the outline column is indented, I'd like to indent the item on the same row in another column by the same indentation amount. (There's also a third column which shows the result of comparing the two items, this column should never be indented.)

Can this only be achieved by subclassing NSOutlineView? And what would need to be overridden in the subclass? Or is there an easier way to get it to indent multiple columns?