This feature is found in the Customization page, which is available to users on the paid Boost plan.
The font selected in this setting will be used for all text on the graph—axis labels, captions, annotations, etc.—instead of the default font (Murecho Variable) supplied by GraphFree.
What Fonts Will Work Well?
You may select any font installed on your local computer. However, not all of the fonts on your computer will work well for mathematical notation. If you plan to use mathematical symbols such as those from the Special Characters panel, you should make sure that the font you choose contains the glyphs (characters) you need. If you try to use characters that are not included in the font you choose, then most browsers will substitute the glyph from a generic font instead. Depending on the two fonts involved, this may create a jarring style mismatch.
How Do I Enter the Font Name?
You must enter the name of the font exactly as it is found on your computer. For example, Comic Sans will not work; you must instead type Comic Sans MS, which is the full name of the font.
The easiest way to determine the "official" name of a font is usually to open a word processor and view the font names there.
Ugh, Why Can't I Just Select the Font From a Menu?
To keep you safe from nefarious actors, browsers have historically made it impossible to read information from a local computer's hard drive. Browser makers have recently started experimenting with letting browsers access your installed fonts, but at the time of this writing, this feature was not yet broadly available.