Why dont Charts allow Formula fields as graphable values?

Why dont Charts allow Formula fields as graphable values?

Hi,
I was trying to create a Chart view and found that you need a number/percent type of field in a form before the Chart button is enabled - which makes sense(!), but shouldnt you also be able to use a Formula field as well? The Chart button's not enabled for me when I have only a formula field in a form. But as long as you ensure the formula result is a number that can be graphed then shouldnt it be enabled also?

Or am I missing something?