The graphs and other figures in Guns, Germs, and Steel certainly clarify and add to the text. Mainly, they do so by summarizing the major points of various chapters in a concise way. A reader can look at the graphs to get the main point of a chapter and then read the text to understand more details.
One example of this is Figure 4.1. This figure shows very clearly what Diamond argues in this chapter. It shows, in graphic form, which ultimate factors led to which proximate factors. Another example is Table 14.1. This table takes the very long discussion of the types of societies from Chapter 14 and boils it down to a version that can be understood at a glance. As these examples show, the graphs in this book help the reader to grasp the main points more easily.