This technical note explores the applications of usability testing to enhance the effectiveness of data visualizations in persuasive communication. It argues that beyond following best practices for visual design, usability testing--a common user research approach in website, app, and software development--can significantly improve how well data visualizations meet audience needs and accomplish persuasive goals. The note details various aspects of usability testing, including its objectives and methodologies applicable to charts versus interactive dashboards (e.g., five-second tests, task-based tests, and think-aloud protocols), and emphasizes iterative design based on feedback. By highlighting the importance of both qualitative and quantitative feedback from representative users, the document provides insights into creating more intuitive, efficient, and impactful data visualizations.