en/2007/09/draw-a-map-of-china-onto-a-color-image/

What we need to do is just use the parameter add = TRUE in the function of map() (package maps):
[…] […] Source code:
[…] if (require(maps) && require(mapdata)) { par(mar …

https://yihui.org/en/2007/09/draw-a-map-of-china-onto-a-color-image/