I’m pretty happy with the Google Charts API but in case you’re looking for even more graphing solutions for your website, there are two that are more advanced:
PlotKit is a Chart and Graph Plotting Library for Javascript. It has support for HTML Canvas and also SVG via Adobe SVG Viewer and native browser support.
Flotr comes from Plotr, which itself was derived from Plotkit, here’s the official description:
Flotr is a javascript plotting library based on the Prototype Javascript Framework (version 1.6.0.2 at the moment) and inspired by Flot (written by Ole Laursen).
Flotr enables you to draw appealing graphs in most modern browsers with an easy to learn syntax. It comes with great features like legend support, negative value support, mouse tracking, selection support, zoom support, event hooks, CSS styling support and much more.
Both are nice solutions, PlotKit itself derived from an older library called CanvasGraph. If you know others, do mention them in the comments.