Are you talking about creating graphs on-the-fly, or displaying static graphs that you change only once in a while? PHP has the "GD" graphics library, which can be used to create a picture or graph when the page is displayed. I don't think GD has graphing functions built in; you'll have to find or write a library to take your data and turn it into GD commands.
If this is just updated once in a while, you would probably be best off finding something that runs on a PC (such as a spreadsheet) to create your graphs, capture them as .gif files or .svg files, and upload them to your site. Use an image editor such as GIMP or PhotoShop to embellish the text and labels. For such one-off uses, I wouldn't bother trying to find something that runs on the site -- it's more effort than it's worth.,