Click on a plot and then choose Copy Plot button on Mini toolbar

E.g. on the left graph below, click the continent border (black plot plot) and click Copy Plot button.  Then on the right graph, Ctrl+V. Border lines will be added. 

Labtalk command

layer.plot.copy() //copy active plot or group of plots

layer.plot2.copy() //for ungrouped plot, copy 2nd plot