On a report with graphs, it’ll show the graphs in the html view, but when the same report is made in PDF format the graphs won’t show. This bug is caused by the PHP configuration (normally this file is found in /etc/php5/apache2/php.ini), with this configuration you have to disable the flag to make the session pass by as a parameter. Well, for setting it on the flag, you can change the line:
session.use_only_cookies = 0
The goal is to create the PDF, making the server auto-fetch the image graph and use the user session as a parameter.