  While a number of touch-based visualization systems have appeared in recent
years, relatively little work has been done to evaluate these systems. The
prevailing methods compare these systems to desktop-class applications or
utilize traditional training-based usability studies. We argue that existing
studies, while useful, fail to address a key aspect of mobile application usage
- initial impression and discoverability-driven usability. Over the past few
years, we have developed a tablet-based visualization system, Tangere, for
analyzing tabular data in a multiple coordinated view configuration. This
article describes a discoverability-based user study of Tangere in which the
system is compared to a commercially available visualization system for tablets
- Tableau's Vizable. The study highlights aspects of each system's design that
resonate with study participants, and we reflect upon those findings to
identify design principles for future tablet-based data visualization systems.
