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Mathieu Jacomy added introduction bloc 2.tex
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Before we move to the enunciation of the visual grammar of networks, however, we would like to briefly discuss the reasons that have delayed so far this type of reflection. These reasons date back to the very foundation of graph mathematics. In solving to the problem of Königsberg’s bridges, Euler performed the most classical of mathematics operations. He abstracted the formal structure of the problem from its empirical features: he took a city and turned it into a table of number (see figure 1). In doing so, Euler laid the foundation of discrete mathematics at the cost of separating the idea of network from its physical materializations. His operation has been so successful that, for the following two centuries and a half, the reflection on networks was dominated by their structural properties, with little interest for practical applications. One of the consequences of such focus on structures (at the expenses of the actual contents of networks) has been that mathematicians never saw the interest of representing networks. For them, design a network was (and still is) perfectly useless.
FIGURE 1
The idea that it could be worth to draw a network to see what it looked like came from a different tradition: the tradition of social networks analysis. Jacob Moreno, founder of this approach, was very explicit about the importance of visualization: “A process of charting has been devised by the sociometrists, the sociogram, which is more than merely a method of presentation. It is first of all a method of exploration. It makes possible the exploration of sociometric facts” (1953, pp. 95-96). Since Moreno and his followers were working on real networks constructed by the observation of actual social relations, it made sense to them to design networks. Identifying visual patterns became the equivalent of looking for social dynamics. In a seminal paper published on the New York Time in 1939, Moreno refers to network analysis as “a new geography” (see fig. 2). By drawing his sociograms, Moreno reconnected networks with their most ancient ancestor: the geometric figure, a set of points and line whose properties are meant to be explored by its visual representation. Graphs returned to be graphic.
FIGURE 2
This of course, was only the beginning of the reflection on the visualization of networks. Once you decide to draw a network as a set of points and lines, you still have to decide which colors to use, which stroke, which style and, most crucially, which composition rules to follow. None of these questions is trivial and the story of how early analysts of social networks wrestled with their design is long and interesting. Unfortunately, we do not have here the space to tell such story (but see Freeman, 2010 for a remarkable account of it). For the paper, it will be enough to remark that, though crucial for the founders of social network analysis, the reflection on network design progressively lost its interest for their followers. Understandably fascinated by the parallel developments of graph mathematics, later social networks’ analysts focused on statistics and progressively neglected networks design. In this paper, we draw on the reflection of Moreno and his early followers to discuss how the visualization of networks can be exploited for the study of social phenomena.
The next paragraph will introduce the three main visual variables \textbf{(ADD REFERENCE TO BERTIN)} mobilized by visual network analysis: the position of nodes, their size and their color. We will briefly discuss the features of each of these variables and we will describe how they are employed to represent different characteristics of networks. After having set the theoretical basis of our approach, we will then propose an example of analysis in order to provide a practical guideline for visual network analysis.
