[R-sig-Geo] Use of formula in spplot function

[Paulo van Breugel]

Dear all,

I am exploring the options of plotting data using the sp package. It is, 
after some trial and error, not too difficult to produce nice looking 
maps combining polygon, line and grid data with spplot. One thing I 
haven't been able to figure out though is the use of the argument 
'formula' in the spplot.grid function. Does anyone has a good example 
explaining its use?

Best regards

Paulo van Breugel