I love the Google charts API. It has saved me from relying on large (and usually buggy) PHP graph libraries in a couple of projects. PHP graph libraries tend to need quite a bit of configuration before data can be displayed, which means it takes a lot of work to you have to move to a different library. With this all you need is a little array to string conversion :)
One other great thing about Google graphs is that you can use JavaScript to change the source of the graph image and effectively create a dynamically changing graph.