fedplot was designed with a specific set of fonts in mind.
You can use the install_fed_font() and
load_fed_font() to install a free version of these fonts.
Details
Note that these functions are currently only partly implemented across
operating systems. For instance, install_fed_font is only implemented
for Linux, and load_fed_font for Windows.
Lastly, note you can also override the font used through options(), such as in:
options(fedplot.font_family = "Times New Roman")