One of the most difficult parts of any graphics package is scaling, converting from data values to perceptual properties. The inverse of scaling, making guides (legends and axes) that can be used to read the graph, is often even harder! The idea of the scales package is to implement scales in a way that is graphics system agnostic, so that everyone can benefit by pooling knowledge and resources about this tricky topic.
The scales package is made up of the following interdependent components
Palettes, pal for short, describe the useful palettes of aesthetics.
Transformations, trans for short, describe common scale transformations,
their inverses, and ways of generating breaks and labels.
Bounds: various ways of rescaling the data
Scaling functions: pull together palettes, bounding functions and
transformations to provide a complete pathway from raw data to perceptual
Mutable ranges: in many graphics pathways, scale ranges can not be
computed in a single pass, but must be computed over multiple groups or
multiple panels. The mutable ranges (implemented with R's new
reference based class) provide a thin layer of mutability to make this task
Breaks and formats: ways of computing how tick marks/legend keys should be
distributed across the data range, as well as how to convert those numeric
positions into reader-friendly labels