For me on Debian Linux and eGroupWare Version 1.6.001 and Jpgraph 2.3.3 helped commenting one string in file projectmanager/inc/class.projectmanager_ganttchart.inc.php function text_encode($text):
In eGroupWare settings (etemplate/process_exec.php?menuaction=projectmanager.projectmanager_admin.config)I set encoding to 'utf-8'.
After that with default font ('Vera') I see squares instead of russian letters. But setting the font 'arial.ttf' for Jpgraph helps me.