With notions of data visualization expanding to include contemporary art and design, data visualization represents an important new dimension for transdisciplinary art education. The pedagogical potential of these practices has begun to be recognized in many fields, including art education. However, despite substantial interest, little research examines the ways K-12 students are engaging in arts-based data visualization. As part of the Arts-Based Data Visualization Project, we conducted research examining middle school students' use of signs and visual metaphors to communicate ideas about social and ecological data and developed a diagramming model to represent these semiotic relationships. This study demonstrates that, when generating arts-based data visualizations, middle school students employ a wide range of semiotic approaches, some of which can be original, complex, and intriguingly ambiguous. However, as visual metaphors were present only in a modest portion of the student data visualizations, these metaphors, if sought, will need to be explicitly cultivated.