This compilation thesis explores how the double aspect of visual literacy is enacted in secondary schools' social science classrooms when interactive data visualizations are employed. The aim is to map what characterizes 'reading' interactive data visualizations and 'writing' knowledge visualizations, as well as implications for a didactic design supporting students' visual literacy. The interactive data visualization introduced by this thesis is the visual analytics application Statistics eXplorer, which offers support in analyzing vast amounts of data. Such information-rich interfaces provide possibilities for students to find correlations and draw conclusions, but they also generate complexities regarding interactive and multimodal texts and require modes other than the written when insights are to be demonstrated. The thesis is positioned under the umbrella of actor-network theory, thereby a socio-material perspective guides the study of interactions between actors (students, teachers, lesson plans, visualizations, written texts, etc.) Applying design-based research, an intervention is designed and conducted in seven social science classrooms comprised of four teachers and 152 students. The empirical material consists of zoomed-in webcam recordings capturing the students' faces, voices, and gestures as well as the activities on the screens. It also includes wide-angle captures, field notes, and one focus group. Material discursive analysis guides the analytical work. The findings reveal a reading characterized as intense, performative, collaborative, and dynamic. The reading process is distinguished by searches for a starting point, a production of reading direction, and a continuously changing reading surface. The findings also show how students' insights from this kind of reading can be translated into knowledge visualizations in a writing process that is characterized by exploring, gathering, and inserting visuals as carriers of information. Furthermore, by identifying critical issues in the classrooms, a didactic design framework of vital components is constructed, which demonstrates how teachers can design teaching that supports the development of students' visual literacy.