Then, since the end-user of any information visualization system is a human being with specific perception capabilities, another aspect to take into account is all the knowledge we have from the psychology of perception, because if we design a visualization and this visualization does not take into account the basic knowledge about visual human perception, our visualization might be cleverly designed, but it might be difficult for the user to perceive subtle differences in the visualization itself. So, it is a clever visualization, but human beings are not good at perceiving differences in it, so it is unuseful for a real application. Another important aspect of human factors is the metaphors we are using, because when we choose to use a metaphor in our visualization we must be aware that the user will try to apply to the visualization the mental models he has. For example, if the program uses a lens metaphor, and we draw a lens on the screen, the user will try to apply her mental model of a lens and will try to take the lens and put in on the data and maybe he tries to see if the lens magnifies the data and if that does not happen the user can be disoriented.