This paper is based on a study which explored the conceptual understanding of Grade 11 mathematics learners of graphical functional relationships, in particular their understanding of the Cartesian plane, notation, symbols and graphical terminology. The National Curriculum Statement (NCS) for Grades 10-12 in mathematics stipulates functional relationships as a key learning outcome, emphasising a variety of graphical skills. The study highlights visualisation skills in graphical interpretation, and explores the relationship between the way graphs are presented and learners' understanding of graphs. Learners' understanding of graphical representations is analysed using the procedural-conceptual dichotomy. Grade 11 learners, who had engaged with graphical representations in Grade 10, were interviewed in two schools. This research indicated a strong tendency of these learners to use procedural knowledge to explain simple concepts and, further showed that the learners' visual understanding was weak and dependent on the diagrams that educators use in the classroom.