Treemaps were introduced by Ben Shneiderman in 1991. A treemap displays hierarchical tree-structured data as a set of recursively subdivided rectangles. In other words, it displays each branch of the tree as a large rectangle, which is then tiled with smaller rectangles representing subbranches. This process continuously repeats itself till it reaches the leaves of the tree.

Before we dive into the code example, let's first define what we mean by hierarchical data. So far, we discussed many types of visualizations capable of representing flat data structure usually stored in one or two dimensional arrays. In the rest of this chapter, we will switch our focus onto another common type of data structure in data visualization-the hierarchical data structure. Instead of using arrays, as in the case of flat...