As ideas from data science become more prevalent in secondary curricula, it is important to understand secondary teachers' content knowledge and reasoning about complex data structures and modern visualizations. The purpose of this case study is to explore how secondary teachers make sense of mappings between data and visualizations, especially depictions of multivariate relationships. The participants were 14 in-service secondary teachers who were video recorded as they worked through three sets of activities. In these activities, participants created a visualization (network graph) from multivariate data, encoded raw data for several attributes from visualizations depicting multivariate relationships, and structured data into a tidy format. With minimal instruction, participants were able to create visualizations when given data representing multivariate relationships. They were also able to structure non-tidy data into a tidy format with some scaffolding and discussion. Notably, creating data tables from visualizations, especially relational tables, seemed more challenging for them. These results provide insight into secondary teachers' reasoning about connections between multivariate data and visualization.