NCES Kids' Zone

The U.S Department of Education's National Center for Education Statistics (NCES) offers Kids' Zone. It provides information to help students learn about schools; decide on a college; find a public library; engage in several games, quizzes and skill building about math, probability, graphing, and mathematicians; and to learn many interesting facts about education.

This webpage on graphs and charts offers a useful tutorial on displaying data and conveying information effectively:
http://nces.ed.gov/nceskids/createagraph/default.aspx.