Linear functions are an essential part of school and university education. Nevertheless, this topic is challenging for many students--especially in STEM topics. In this article, we contribute to the understanding of learning difficulties in the context of mathematical and physical problems. Here, we present the results of an eye-tracking study on visual attention during the interpretation of linear graphs using the gaze-cued retrospective think-aloud (RTA) method. Gaze data of N=131 and interview data of N=60 grammar school students were recorded while solving items of a validated test instrument: pairs of items in a mathematical and kinematic context that are isomorphic, i.e., have similar surface features and require the same procedure to solve. We will show that the difficulties in interpreting the kinematic diagrams, which have already been identified in 9th-grade students, are still present in students in the entry phase of the upper secondary level. Further, students' eye-tracking behavior differs significantly between mathematical and kinematic contexts. Triangulation of eye-tracking data with RTA data reveals context-dependent solution strategies--transfer from mathematics to physics is a major problem for students.