Did you set $g_system_font_folder to the location where your font files are located?

I played around with font folder / names. I haven't checked the code.

$g_system_font_folder = '/System/Library/Fonts/';
$g_graph_font = 'Courier.dfont';
$g_font_per_captcha = 'Courier.dfont';
#$g_graph_font = 'Thonburi.ttf';
#$g_font_per_captcha = 'Thonburi.ttf';

The GD libraries don't handle Mac fonts directly. The best bet is to download a set of TrueType fonts and reference them using the "system_font_path". You may need the fix in bug# 10694 to make jpgraph work.

See http://corefonts.sourceforge.net/ for details on getting Arial, Verdana, et al.

Mac truetype fonts can be downloaded from: http://support.uni-klu.ac.at/PhpKurs/0300_JpGraph_Bibliothek
$g_system_font_folder should be pointed to the folder that contains the truetype fonts.