So, a general word about mapping. Two questions that an IV designer should ask himself or herself when designing, when approaching the mapping phase of his project. First he must be aware that the mapping between elements and relations in the dataset and the elements and relations in the visualization has to be always found in such a way that the aspects which are conceptually important in raw data, in the dataset, are made perceptively important by the visualization. So, what is conceptually important has to be immediately, easily perceived by user. It seems almost trivial, but unfortunately IV designers often pursue the goal of providing the system with very impressive graphics, graphics that stand out, but in the pursue of this goal they forget this very important principle, so they produce graphics that are not very intuitive, that are just impressive. But impressive graphics do not allow you to make good decision if it does not follow this principle. And then a second general principle that comes from HCI, but here it is applied to the specific case of mapping is that you have to ask yourself if your mapping has always been implemented in your system in a precise and consistent way. For example, if you use colour coding then you must be aware that your colour code is always followed in the same way, in every alternative visualization you propose on the same screen. Otherwise it would become very easy for the user to get confused. The user would naturally expect that the same colour code is used in every different visualization on the same screen. So these two general principles correspond to two questions that the IV designers should ask himself in every moment of the design of a mapping.