In this chapter we are going to cover one of the most fascinating aspects of D3: force. Force simulation is one of the most awe-inspiring techniques that you can add to your visualization. Through a number of highly interactive and fully-functional examples, we will help you explore not only the typical application of D3 force (for example, the force-directed graph), but also other essential aspects of force manipulation.

D3 force simulation support was created not as a separate capability, but rather as an additional D3 layout. As we have mentioned in Chapter 9, Lay Them Out, D3 layouts are non-visual data oriented layout management programs designed to be used with different visualization. Force layout was originally created for the purpose of implementing a specific visualization type called force-directed graph. Its implementation uses standard verlet integration based particle motion simulation with support for simple constraints.

In other words, D3 implements a numeric method that is capable of loosely simulating Newton's equation of motion on particle level and with simple constraints simulated as links between particles. This kind of layout, of course, was ideal in implementing a force-directed graph; however, we will also discover through recipes in this chapter that force layout is capable of generating many other interesting visualization effects due to its flexibility in custom force manipulation. The application of the techniques introduced in this chapter go even beyond the data visualization realm and has practical applications in many other domains, for example, user interface design. Of course, we will also cover the classical application of force layout: the force-directed graph in this chapter.