By way of analogy, consider a chart of somebody's body temperature in Fahrenheit: if that fluctuates +/- 5 degrees it's a cause of significant concern, +/- 10 degrees is a medical emergency. But all of those changes would look small and insignificant on a 0-based chart.

It's the same situation here. The ASP of a Mac is never going to be even close to 0, so the Y-axis should span only the range of realistic values so we can really see how significant the recent upswing was in context.