Data-Ink Ratio

Overview

The data-ink ratio measures how much of a graphic's ink is doing real informational work. Tufte's core claim in The Visual Display of Quantitative Information (Chapter 4): the larger the share of ink devoted to data, the better the graphic, other things equal. The ratio is not a grade to chase but a lever — it tells you to find every mark that could be erased without losing data, then erase it. This dissolves the chronic problem of decoration, redundancy, and convention adding ink that competes with the data instead of conveying it.

§1. The Ratio: One Definition, Three Equivalent Forms

Data-ink is the part of a graphic that responds to the numbers. Tufte's canonical definition:

"Data-ink is the non-erasable core of a graphic, the non-redundant ink arranged in response to variation in the numbers represented." — Tufte, The Visual Display of Quantitative Information

The ratio has three equivalent formulations. The skill must carry all three, because each exposes a different lever:

(1)  data-ink ratio = data-ink / total ink used to print the graphic

(2)  = the proportion of a graphic's ink devoted to the
       non-redundant display of data-information