Problem solved. Just add the following lines after the new GD::Graph::linespoints object is created. The number next to the font defines the font-size property.

Of course, unifont.ttf must be a TrueType font file supporting unicode.

$unifont = "./unifont.ttf";
$my_graph->set_title_font($unifont, 18);
$my_graph->set_legend_font($unifont, 12);
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For more methods, visit GD::Graph.