By default in 0.8.7 and later of Cacti, Guest or Anonymous access is disabled. This is shift from previous release that came with the "Guest" user enabled and set.
Enabling "Guest" access is easy. But, you must understand that you should only allow the designated "Guest" user access to what you authorized. It is not sugguested that you give the designated "Guest" user more than just access to graphs. To enable guest access, first, need a designated "Guest" user. Cacti by default comes with one already created that has access to only graphs. It is important to note that the default guest user has access to all graphs on the system. Once you have created the "Guest" user, you need to tell Cacti what the "Guest" user is. This is done by updating the Authenication Settings and selecting the "Guest" user you have designated. The "Guest" user must be enabled and be set as the guest user for anonymous access to work.