你如何使用gnuplot的内置字体？

gnuplot文档有关于字体的说法:

    Five basic fonts are supported directly by the gd library. These are
    `tiny` (5x8 pixels), `small` (6x12 pixels), `medium`, (7x13 Bold), 
    `large` (8x16) or `giant` (9x15 pixels).

但是,当我尝试使用一个:

    gnuplot> set terminal png font tiny

我明白了:

    Could not find/open font when opening font tiny, using default

我如何使用这些看似内置的字体？