I was configuring a new server to serve Cacti recently and found that the graphs were not showing. After much troubleshooting and configurations changes, I found out that it was due to disabled functions in the PHP.
Check if popen is inside the PHP disabled_functions settings. Remove it if it is.
I was using aaPanel on CentOS 7 and the disabled_functions were populated with large list of functions.
Cacti Version: 0.8.8h
PHP Version: 5.4