In science, graphs are used to visualise data, relationships and scientific principles. Basic graph reading operations, i.e. reading single data points, recognising trends as well as conducting small extrapolations from the data are important skills for pupils and lay the groundwork for a comprehensive understanding of data visualisations across all disciplines. To explore the relationship between question level and item difficulty, we analysed the graph reading skills of eight-grade pupils in German Grammar Schools (N = 198) with Item Response Theory. In this study, pupils were asked to identify data points, trends and make small extrapolation from realistic descriptive graphs, used as teaching material. Furthermore, we examined the relationship between graph reading skills and motivational/emotional factors as well as academic achievement. Results show that the eight-graders mastered the basic abilities well and -- contrary to our expectation -- are as well able to extrapolate as they are to read trends or data points. We also find some relationship between graph reading skill and academic achievement and motivational/emotional factors as well as content knowledge.