Datafication of student learning has carved out an influential space for public and private actors who design technologies for visualizing data. As data visualizations shape how teachers' interpret data, they are powerful devices. This paper examines how teachers get configured as data users in the making of Danish national test data visualizations for municipal primary and lower secondary schools. The paper is based on a qualitative study of the Danish Ministry of Education, which develops the official visualizations, and NordicMetrics, a private consultancy offering a supplementing visualization of student progression. We draw on science and technology studies (STS) to theorize techno-organizational dynamics of developing visualizations. We propose to understand data visualizations as contingent, situated and socio-material achievements that configure teacher as data users. Comparing two institutions' respective negotiations of different concerns when developing data visualizations enables us to consider the otherwise 'hidden' data mediators and the entwined relations between public and private data mediators.