As Tufte explains, the "layering of data, often achieved by felicitous subtraction of weight, enhances representation of both data dimensionality and density on flatland" (60). I interpret this to mean that layering both adds and subtracts. Layering adds dimensionality to data that is otherwise dull, thus making the info more appealing to the reader. Layering subtracts from the heaviness of data, which makes the information less exhausting to read. It all comes down to how the info is organized. If layering is utilized correctly, the user will walk away with the necessary information, no matter how abundant or scarce it is.

This is an example of layering adding dimensionality and density.
http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja...

This is an example of ineffective layering:
http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja...