g(Math) or the Wizkids CAS Add-ons for Google Docs.
g(Math) for Google Docs offers other features in addition to a graphing. Besides creating graphs you can insert handwriting entries and statistical displays. g(Math) also supports voice input. Extensive tutorials for g(Math) can be found on the g(Math) Help website.
Wizkids CAS is another Google Docs Add-on that offers a graphing calculator feature. Wizkids CAS offers tools for solve=ing equations and plotting graphs, finding numerical and exact solutions, and simplifying and factoring expressions with variables. The video embedded below provides an overview of the features in Wizkids CAS.