Depending on your version of GD, this accepts both GD builtin fonts or the name
of a TrueType font file. In the case of a TrueType font, you must specify the 
font size. See the GD::Text manpage for more details and other things, since all
font handling in GD::Graph is delegated to there.

Examples:

    $my_graph->set_title_font('/fonts/arial.ttf', 18);
    $my_graph->set_legend_font(gdTinyFont);
    $my_graph->set_legend_font(
        ['verdana', 'arial', gdMediumBoldFont], 12)

(The above discussion is based on GD::Text 0.65. 
Older versions have more restrictive behaviour).