I like the definition of true fans:

A True Fan is defined as someone who will purchase anything and everything you produce. They will drive 200 miles to see you sing. They will buy the super deluxe re-issued hi-res box set of your stuff even though they have the low-res version. They have a Google Alert set for your name. They bookmark the eBay page where your out-of-print editions show up. They come to your openings. They have you sign their copies. They buy the t-shirt, and the mug, and the hat. They can't wait till you issue your next work. They are true fans.

There is another way to draw it.  Long tails are typically used to describe the product diversity, pyramids are typically used to show the power purchasers.  I've seen stats lying around the Interweb that 10% of power purchases make up 90% of all online consumer purchase dollars spent.