In this paper we analyze the graphs produced by 207 pre - service primary school teachers when solving an open - ended task in which they should compare three pairs of distributions. The graphs produced are classified according whether they are correct or not, and according to the kind of error in case of an incorrect graph. The analysis allows us to explain some of these errors in terms of semiotic conflicts. Results suggest the semiotic conflicts are related to knowledge of conventions in building the graphs, selecting the graphs, number sense and conceptual misunderstanding. The influence of using computers on the errors produced is also analyzed.