Everyday, we look at dozens of websites. The structure of these websites is defined in HTML. Your browser's job is to render the HTML according to the specs. HTML consists of so-called tags, like the 'A' tag for links, 'IMG' tag for images and so on. Since tags are nested in other tags, they are arranged in a hierarchical manner, and that hierarchy can be represented as a graph. Here's The Generator Blog as a graph. Colors used are blue for links, red for tables, green for the DIV tag, violet for images, yellow for forms, orange for linebreaks and blockquotes, black for the HTML tag, and gray for all other tags.
If you want to see what your site would look like as a graph, go here.