Interpreting statistical graphs and making inferences based on the graphs are a precursor for formal statistical inferences. To support student inferences, both teachers and future teachers should have adequate knowledge regarding students' thinking on graphs as well as their potential misinterpretations and difficulties in interpreting graphs. In this qualitative case study, I examined a preservice middle school mathematics teacher's knowledge of student thinking about interpretation of line graphs. During her practicum course, preservice teacher planned a lesson to teach "creating and interpreting line graphs." I first interviewed with her on the lesson plan and then observed her teaching the subject. The data indicated that the preservice mathematics teacher's actions regarding knowledge of student thinking before and during the lesson supported the students in interpreting graphs beyond reading. Preservice and in-service teachers with more knowledge of student thinking would provide teaching practices to support students' learning of statistical graphs. I thus suggest that a curriculum may be designed for improving preservice mathematics teachers' knowledge of student thinking about statistical concepts and informal statistical inferences.