A series of examples from experiences taken from experiments that have taken place in The Netherlands and the USA were discussed. This work has been carried out by the Research Group on Mathematics Education of Utrecht University, with the USA work a collaboration with the National Center for Research in Mathematical Sciences Education of the University of Wisconsin in Madison. Materials were developed on the subject of Data Visualisation. This subject treats skills such as drawing basic plots and calculating basic numerical summaries of data, but it also treats questions such as the following as being of at least equal importance. What graphical representation is best for this set of data? What do these data tell me? How can I communicate the message through a picture? This approach has implications for testing and evaluation as well as teaching.