Joint attention is widely recognized as an important developmental milestone for children, and experts consider a lack of joint attention a defining characteristic of autism spectrum disorders (ASDs). While clinicians and researchers agree on the importance of joint attention, their definitions and methods for assessing joint attention vary. In this paper, we present the design process and the evaluation of EnGaze, a visualization-based Web tool for dyadic communicative behavior that highlights commonly discussed features of joint attention. While such visualization styles are not yet the norm in the clinical practices of behavioral and developmental psychology, we argue they should be and find that the introduction of these visual artifacts helped clinicians and researchers conceptualize their personal joint attention rules. Researchers envisioned a number of uses for EnGaze in their personal workflow, including identifying atypical communication patterns and providing a visual record for tracking behavior. The contributions of this paper are 1) an interactive visualization for exploring joint attention, 2) the documentation of an iterative design process for a clinical visualization tool, 3) illustrations of EnGaze use cases with active practitioners in the behavioral sciences, and (4) the discussions surrounding the implications of introducing such visualizations in behavioral science communities.