Google has a graph-making facility called Google Chart. The graphs are very nice but the API is not intended for direct use by humans. This page is a front-end that lets you access Google Chart via a nicer point & click interface.
The graph updates itself automatically as you change the settings and data. Once you have your graph set up, you can copy the URL at the bottom into your own page and get the same graph there - or just save the image file.
ACME Labs / Chartmaker