[R] User defined grid on map

[Adriana Bejarano]

This may be a trivial question for some of you: Is there a way to add 
defined grid sizes on specific areas of a map?. I want to add 10kmx10km 
grids to all California coastal counties:

"california,alameda", "california,contra costa", "california,del norte", 
"california,humboldt", "california,los angeles", "california,marin", 
"california,mendocino" ,"california,monterey", "california,napa", 
"california,orange", "california,riverside", "california,sacramento", 
"california,san benito", "california,san bernardino", "california,san 
diego" , "california,san francisco", "california,san joaquin", 
"california,san luis obispo", "california,san mateo", "california,santa 
barbara", "california,santa clara", "california,santa cruz", 
"california,siskiyou", "california,solano", "california,sonoma", 
"california,sutter", "california,trinity", "california,ventura", 
"california,yolo"