Redundant is Not Redundant: Automating Efficient Categorical Palette Design Unifying Color & Shape Encodings with CatPAW

TL;DR

This paper investigates the effectiveness of redundant color and shape encodings in categorical scatterplots, demonstrating improved accuracy and providing a design tool for optimal palette creation.

Contribution

It systematically evaluates redundant color-shape combinations, revealing their benefits and interactions, and introduces a practical palette design tool based on empirical findings.

Findings

Redundant encodings improve class correlation assessment accuracy.

Strong interaction effects exist between color and shape choices.

The palette design tool helps create effective categorical visualizations.

Abstract

Colors and shapes are commonly used to encode categories in multi-class scatterplots. Designers often combine the two channels to create redundant encodings, aiming to enhance class distinctions. However, evidence for the effectiveness of redundancy remains conflicted, and guidelines for constructing effective combinations are limited. This paper presents four crowdsourced experiments evaluating redundant color-shape encodings and identifying high-performing configurations across different category numbers. Results show that redundancy significantly improves accuracy in assessing class-level correlations, with the strongest benefits for 5-8 categories. We also find pronounced interaction effects between colors and shapes, underscoring the need for careful pairing in designing redundant encodings. Drawing on these findings, we introduce a categorical palette design tool that enables designers to construct empirically grounded palettes for effective categorical visualization. Our work advances understanding of categorical perception in data visualization by systematically identifying effective redundant color-shape combinations and embedding these insights into a practical palette design tool.