There is a very large gap in time (x-axis on the plot) for units less than 3.2 by old!
Both the Copernicus and Tycho age dates are inferred.
Simply stated, the older an area the more craters you will find.
How do scientists determine the absolute age of a geologic unit?
The x-axis is the age of the surface, in billions of years. The number of craters greater than a given size per unit surface area (usually million km) provides a relative age for that surface.Heavily cratered surfaces are old, and sparsely cratered surfaces are young.New crater counts acquired from LROC WAC and NAC images and Kaguya images reveal a different crater density for Copernicus! Scientists still have to live with the sparsity of absolute age points that define the chronology curve, for now. To really nail down the connection between crater density counts and absolute ages we need samples of middle aged and young mare (3 by to 1.5 by years).