This function draws a (line) border around regions of the
plotting area of a Plot which correspond to valid, unclipped
physical coordinates. For example, when plotting using an
all-sky map projection, this function could be used to draw the
boundary of the celestial sphere when it is projected on to the
plotting surface.
If the entire plotting area contains valid, unclipped physical
coordinates, then the boundary will just be a rectangular box
around the edges of the plotting area.