Parallel Coordinates Plot

CORE DEFINITION

Parallel coordinates plots place one vertical axis per variable and draw each observation as a polyline joining its values across all axes, a technique popularised by Alfred Inselberg. Parallel sets apply the same parallel-axis layout to categorical data, showing flows and proportions between categories in the manner of a Sankey diagram. The core proposition is that patterns invisible in two-dimensional scatter plots, such as clustering, inverse relationships and outliers, become readable as the geometry of line bundles. The key qualification is that readability degrades quickly with axis count and crossing lines, so ordering, scaling and interactive filtering are essential rather than optional.