xyplot: logarithmic y-axis

This should be simple but I am struggling. I like to easily switch in xyplot
between a linear or logarithmic y-axis by setting a logical flag logY to
False or True. This switch changes the scales argument of xyplot. I found
out that the original two-dimentional data (Conc vs Time in my case) are
converted to log10(Conc) if log=TRUE in scales, but it appears that
functions like panel.curve need to provide the y values in log10 form (if
there is an automatic method, I would like to know). I therefore like to
pass on the value logY to my custom panel.curve function. How do I do that?
I think that the value of logY should go into xyplot that should pass it on
to the panel function and then to the panel.curve function.