A Treemap displays hierarchical data as a set of nested rectangles. Each group is represented by a rectangle, which area is proportional to its value. Visit data-to-viz.com for more theoretical explanation about what it is. For a R implementation, see below.
Treemap package
        
          The treemap package is probably the best way to build
          treemaps in R. The 3 examples below will teach you how to build a very
          basic treemap, how to deal
          with subgroups, and how to
          customize the figure. Note that
          once you master this package, you can very easily build an interactive
          version as described below.
        
          Using the treemap package, with the right packages and
          data allows you to build very interesting treemaps. Here are some
          examples:
        
d3treeR
          The d3treeR allows to build interactive treemaps with
          R. Click on a group to zoom in and reveal subgroups.
          Click on the group name on top to unzoom and come back to the previous
          state. Note that the syntax used in previous charts above is exactly
          the same. Only one more line of code is needed, give it a go!