"We find out the total number of web pages (using Yahoo Web Search web services) containing the term "Tuesday" for example. Let's call that A. Then we find out how many pages have both "Tuesday" and "hungry", and we call that B. The fraction B/A is then plotted on the graph above Tuesday..."
Just like any other search tool on the Net, Yahoo has some nifty shortcuts that you should take advantage of in order to narrow down your search results and save time.