It’s probably this, but i can’t test, place in your config.php
$config['overlib_defaults'] = ",FGCOLOR,'#000000', BGCOLOR, '#e5e5e5', BORDER, 5, CELLPAD, 4, CAPCOLOR, '#555555', TEXTCOLOR, '#3e3e3e'";
and play with the colors, any luck?
You might also want to check this out Graph legend text color