[derekp7]

I would say that stability refers to purchasing power, and doesn't need to be a flat line. It can also be a simple math function (liner, logarithmic, exponential) which means that it is somewhat predictable. (Edit: I think a better word than "stable" may be "dependable", which is probably what the article meant).

For example, the US Dollar loses purchasing power over time, but that is OK, since it is mostly at a predictable rate. But when something has wild swings up and down, and random times, then it is less suitable to either use as a currency (the stock person with the price gun will be working overtime correcting prices on everything several times a day), and it isn't that good of a store of value if there isn't any reason for it to not lose all of its value over a given period of time.

The Amazon graph you linked to does look fairly stable, a nice exponential function that has very good reasons to remain so for a good period of time (not saying this or anything else is guaranteed, but at least there is something tangible behind that price).