"While the early days of visualization go back over 200 years, actual research to understand how it works really only started in the 1960s. Jacques Bertin’s Sémiologie Graphique (Semiology of Graphics), published in 1969, was the first systematic treatment of the different ways graphical representations encode data. Bertin coined many terms of the trade, such as the mark, which is the basic unit of every visualization, like a bar, line, or circle sector. He also defined a number of retinal variables, which are the visual properties we use to express the data; these include color, size, location, etc."