Like palmer said it's not just numerical data. Numbers has a special duration format which you get when you subtract two dates. You can manually set how you want it presented. E.g. "2d 5h" or just "2d" etc.
I did some digging on the Apple forums. Apparently only one out of all the graph types support the time duration format. I believe it's called the "scatter" chart.
I got a recommendation to convert the duration to days with a function here:
It would be nice if OmniGraphSketcher fully supported duration out of the box though.
I'll send you an email with a .numbers file in it too.