# for quartz device... 
quartzFonts() # lists current mappings 

# Add a mapping for Myriad: 
#    pass in a vector of font filenames in the order: Plain, Bold, Italic, Bold-Italic 
quartzFonts(
   Myriad = quartzFont(
      c("MyriadPro-Regular", "MyriadPro-Semibold", "MyriadPro-It", "MyriadPro-SemiboldIt")
   )
) 

# Now we can call this family in a plot 
plot(c(1:10), c(2:11), family = "Myriad")

Changing fonts in plots

R