[R-sig-Geo] Leader lines for inset map

[Vladimir]

Hello list!

I would like to add leader lines to my inset maps. These lines connect 
the frame of the inset map with the corresponding area on the main map. 
Good examples are here [1], [2], or here [3].

What is the most convenient R approach to do so?

Kind regards,
Vladimir.

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[1] 
http://gce-lter.marsci.uga.edu/public/images/gce_study_area_blowup_nov07_rev.jpg
[2] http://pubs.usgs.gov/fs/fs08203/images/study_area.gif
[3] 
http://www.innovativegis.com/basis/supplements/bm_dec_02/Ironing_Colorado_files/image004.jpg