i had not installed the package corefonts (no arial, verdana and such) on my system but i had the bitstream-vera one. whenever i tried to configure from the plugin configuration page the veramono, courier, etc. nothing changed: i always got the "unable to read/find font" error message. I did add the g_system_font_folder variable in my config_inc.php to point to the bitstream-vera directory (with trailing slash :) ) but nothing. I added a few debug statements and seen that it was always searching for arial.ttf (in the "correct" bitsream-vera folder). i tried adding a link to bitstream-vera.ttf named arial.ttf and graphs started working.