If you already found package extrafont then you probably found how to load and use Windows fonts in R visualizations. But just in case, everything to get started with extrafont is found here and summarized for using fonts in Windows for on-screen or bitmap output below:
One thing to add is a summary of all Windows fonts registered in R. This will come handy when designing new visualizations and deciding on which font or combination of fonts and their faces to use. The code below produces a table where rows are fonts and columns are faces with font name printed using both the font and the face (if available) in each table cell:
The resulting table is this handy visual:
You can download this image or produce your own with the code above.