This graph has Buffers along the horizontal axis and the Physical I/O Ratio along the vertical axis. The values of the physical I/O ratio shown are between approximately 0.1 and 0.5.

The graph of the intuitive values is a smooth curve showing that as the number of buffers increases the physical I/O ratio decreases.

The graph of the actual values shows that at some values (points A,B, and C in the text) the physical I/O ratio stays about the same as the number of buffers increases, then decreases quickly at a higher number of buffers.