In Grapher, text can be edited in the Text Editor dialog or the Property Manager.  Font properties applied in the Property Manager are applied to the full text string.  Font properties applied in the Text Editor are applied only to the selected characters.

To change the properties of a single character or word, highlight the text in the Text Editor. Make any changes to the font properties and the properties are applied to only the selected text. Click OK. 

Note that the font properties applied to individual characters or words in the Text Editor override any properties applied to the full string in the Property Manager.

Highlight only the selected portion of the text to edit.

Updated July 26, 2019