Sometimes it may happen that the CSV export from a Cacti graph is not working for large date ranges. You then may see an error 500 being returned by the browser. If this happens, you should try increasing the memory limit of your php instance.
This can be achieved by editing the php.ini file and changing the following line:
memory_limit = 64M ; Maximum amount of memory a script may consume
Try settings this to 256M:
memory_limit = 256M ; Maximum amount of memory a script may consume
and restart your webserver. On CentOS you can use the following command: