Google has a nice charting API that lets you send data to it and returns an image that you can use on your site.
But some of their formatting can be a little complicated.
This PHP script lets you include charts on your websites powered by the google charts API.
This plugin handles all calls to google and even includes caching.
This plugin is in use on DBStract and Surveyti.me as well as several other projects and serves it’s purpose well.
Includes a sample index.php file that contains a couple different styles of charts to show how to get started.