Background: Data commentary, in-text comments on the visual presentation of data, is acknowledged as a central aspect of academic writing in many engineering disciplines. At the same time, it is a feature that has been shown to be challenging for students. One of the genres in which data commentary plays a significant role in many engineering disciplines is the master's thesis. Comparatively little research has been done on the process of master thesis supervision, and combining the study of data commentary and master's thesis supervision is therefore particularly interesting. Purpose: This study explores the challenges of data commentary writing through interviews with master's students and thesis supervisors of chemical engineering. Sample and method: Master's students at a Swedish university were invited to participate in a workshop about the writing of data commentary. Nine master's students and five supervisors were interviewed about what is difficult and important about writing data commentaries in their discipline as well as about decisions made in data commentaries written by the students. The interviews were divided into a semi-structured and a discourse-based part. Results: Our results indicate that data commentary comes with a variety of challenges. Among the most difficult and important aspects are selection of content and clarity. The study also indicates a close connection between data commentary and disciplinary learning in chemical engineering, suggesting that highlighting data commentary in the teaching of master's thesis writing will be time well spent. Conclusions: In order to make the teaching and learning of data commentary effective in the context investigated, we propose that important measures are: the development of a shared metalanguage among students and supervisors, a genre approach, and collaboration between engineering and communication faculty.