Map Between World and Device Coordinates

USAGE:

device.xy(wx, wy) 
world.xy(dx, dy) 

ARGUMENTS:

dx,dy
vectors of x- and y-coordinates in the current device coordinate system.
wx,wy
vectors of x- and y-coordinates in the current world or user coordinate system.

VALUE:

a list with elements named x and y that give the mapped coordinates, i.e., device coordinates for device.xy and world coordinates for world.xy.