Once upon a time, I changed my ggplot2 font using using windowsFonts(Times=windowsFont(\"TT Times New Roman\")) to change it. Now I can\'t get it
To change all the fonts in your plot plot + theme(text=element_text(family="mono")) Where mono is your chosen font.
List of default font options:
R doesn't have great font coverage and, as Mike Wise points out, R uses different names for common fonts.
This page goes through the default fonts in detail.