From Edward Tufte’s The Visual Display of Quantitative Information:
A large share of ink on a graphic should present data-information, the ink changing as the data change. Data-ink is the non-erasable core of a graphic, the non-redundant ink arranged in response to variation in the numbers represented. Then,
= data-ink / total ink used to print the graphic
= proportion of a graphic’s ink devoted to the non-redundant display of data-information
= 1.0 – proportion of a graphic that can be erased without loss of data-information.