This is partly described in the accompanying text. The illustration is on two axes, salesdate and customerid. In addition:

• salesdate is broken down into 1999Q1, 1999Q2, 1999Q3, 1999Q4, 2000Q1, 2000Q2, 2000Q3, and 2000Q4.
• customerid is broken down into hashes H1, H2, ... H16.

The hash partition H9 is highlighted in the illustration.