This is partly described in the accompanying text. The illustration is on two axes, salesdate and customerid. In addition:
salesdate is broken down into 199Q1, 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.