Q: I want to locate some sub-viewports in a map, that means the viewport center should be set to the precise place with longitude and latitude. Is it possible to use the coordination of another viewport(here the map plot)?
A: the book “R Graphics” led me to the R graph gallery website. I found the solution there.
http://rgraphgallery.blogspot.de/2013/04/rg-plot-pie-over-google-map.html
http://rgraphgallery.blogspot.de/2013/04/rg-histogram-bar-chart-over-map.html
http://rgraphgallery.blogspot.de/2013/04/rg-plot-bar-or-pie-chart-over-world-map.html