OK, well you have quite a few different options. You could use something like RGraph which is a Javascript canvas-based graph library. The only disadvantage is that older versions of IE can't display HTML5 canvas elements.
You could create them as images yourself with GD. In the main, it's just a case of arranging your data and drawing some lines or boxes, the math for that is pretty simple (just think how you would do it if you were drawing it out on paper and you won't go wrong). You could also "draw" graphs using <div> tags and a lot of CSS, which would allow you to create all kinds of interactive tooltips, etc.
Then there are libraries like Pear Graph which create those graph images for you. The package isn't maintained, but generally there's not too much that would need updating in such a library in terms of maintenance.