A question on colors for plotog groupedData

I have a groupedData object named data.
When I use plot(data) I obtain a trellis plot for each group with a grey
bakground. How can I change the background to white? I tried with
par(bg="white") but got no change at all.

I would appreciate any suggestion.

Albert Sorribas
Grup de Bioestad??stica i Biomatematica
Departament de Ci??ncies M??diques B??siques
Universitat de Lledia
tel: +34 973 702 406
FAX: +34 973 702 426
Home page: http://www.udl.es/Biomath/Group