Now, we have to talk about the other subcategories that I introduced at the beginning. So, so far we covered force-directed layout, and more precisely, force-directed layout within the category of node links diagram. Now we have another category, with the node links diagram which is the one on fixed layouts. So, what is a fixed layout? Well, very simply a fixed layout is a layout in which the position of the nodes is fixed from the very beginning according to some criteria, and there are many possible criteria out there. Typically, we have three main categories of fixed layouts. The first one is the circular layout where the nodes are just arranged around the circle and they are equidistant. The second one is the one where the nodes are arranged along a line. The third one is a fixed 2D layout where the nodes are arranged in a grid or variance of a grid. So now let me focus on the first one because that's by far the most common fixed layout that you will see out there. So, from this point on I'm going to talk only about the circular layout as an example of fixed layout. But before I move on, I think it's important for you to keep in mind that that's not the only option out there, there are basically endless possibilities you can be as creative as you want in determining different types of fixed layouts. Now, when is a fixed layout more appropriate than a force-directed layout? I think that's probably the most important question to ask yourself. Why should I use a fixed layout if I have a nice force-directed layout available? I think there are a number of reasons, but above all, first of all, the most important one is that you have to keep in mind that what a force-directed layout achieves is to show the structure of the network. Where structure meaning a lot of things but typically is whether nodes tend to be grouped together and then show connection between these groupings, okay. So this structure. But what is really important to keep in mind is that there are lots of situations out there where structure is not the most important thing you want to visualize or you want to understand in a network. There are situations where the kind of questions that you have about your network are not related to the structure. So, what are some advantages of the fixed layout? Well, the first one is visibility. Typically, since you can arrange the nodes in very specific locations, you don't have these skewed distribution of nodes where you have lots of nodes clamped together in one region, some are sparse, some are clumped together in another region, they typically are positioned in very specific locations and they are equidistant, so you have more leeway to arrange the nodes in a way that you have more visibility, okay? When I say visibility, I mean visibility of nodes, visibility of the edges and visibility of the properties that you may have mapped and coded to the links, to the edges and to the nodes. So, let me show you an example here of the same data set visualized with a force-directed layout and visualized with a circular layout. I want you to think for a moment on what are the main differences. What information is easy to extract from the force-directed layout, and what information is easy to extract from the circular layout, right. Think about it for a moment. I think one of the things that you can notice in the circular layout is that it's much harder to identify structure, but it's also much easier to identify say, large or small nodes or even better nodes that are connected to lots of other nodes or connected to only a few nodes. So, these are pieces of information that is much much easier to identify. There is another big advantage of fixed layouts. The advantage is that since the layout is fixed, you can split the different regions of the layout into different categories, and because of that, as a consequence, as a byproduct of this choice you can see relationships not only between the nodes but also between the categories in which these nodes are grouped which is very, very nice property of fixed layouts. Here is an example with a circular layout. If I can categorize all the nodes into four classes, in this case A, B, C, D, I can see relationships not only between the nodes but also between the classes. Here is a nice example that I have taken from a project of, Moorits Stefano, and here is a connection between different types of mostly products coming from a company. As you can see here he's using exactly the same technique that I've just mentioned. Not only you can see the connection between the nodes, but you can also see the connections between different categories.
