  We describe a technique for bundled curve representations in
parallel-coordinates plots and present a controlled user study evaluating their
effectiveness. Replacing the traditional C^0 polygonal lines by C^1 continuous
piecewise Bezier curves makes it easier to visually trace data points through
each coordinate axis. The resulting Bezier curves can then be bundled to
visualize data with given cluster structures. Curve bundles are efficient to
compute, provide visual separation between data clusters, reduce visual
clutter, and present a clearer overview of the dataset. A controlled user study
with 14 participants confirmed the effectiveness of curve bundling for
parallel-coordinates visualization: 1) compared to polygonal lines, it is
equally capable of revealing correlations between neighboring data attributes;
2) its geometric cues can be effective in displaying cluster information. For
some datasets curve bundling allows the color perceptual channel to be applied
to other data attributes, while for complex cluster patterns, bundling and
color can represent clustering far more clearly than either alone.
