[R] Getting default aspect ratio from lattice

[Waichler, Scott R]

How can I get the value of the aspect ratio that is used in a lattice
plot?  In a levelplot for instance, the native units per cm of my x and
y axes are different, and I need to know the aspect ratio so that I can
correctly plot vectors.  I know how to set the aspect in a high-level
lattice function but I can't quite figure out how to get it.  I would
like call to levelplot() without printing anything, get the aspect
ratio, use it to create my vector arrows, then call levelplot() again
with print() to create the final figure.

Scott Waichler
Pacific Northwest National Laboratory
scott.waichler _at_ pnl.gov